[AMDGPU] Assert that we can find subregs in copyPhysReg. NFC. (#70332)

This helped to catch a codegen failure caused by #69703. MachineVerifier
did not complain about this malformed COPY either before regalloc:

  %9:vreg_64 = COPY %17:vgpr_32

Or after regalloc:

  renamable $vgpr0_vgpr1 = COPY renamable $vgpr2, implicit $exec

But we can at least catch the problem when copyPhysReg tries to expand
it into 32-bit register moves and fails to find suitable source
registers:

  $vgpr0 = V_MOV_B32_e32 $noreg, implicit $exec, implicit-def $vgpr0_vgpr1, implicit $vgpr2
  $vgpr1 = V_MOV_B32_e32 $noreg, implicit $exec, implicit $vgpr2, implicit $exec

Author: jayfoad

llvm/lib/Target/AMDGPU/SIInstrInfo.cpp

@@ -742,23 +742,27 @@ static void expandSGPRCopy(const SIInstrInfo &TII, MachineBasicBlock &MBB,
 
   for (unsigned Idx = 0; Idx < BaseIndices.size(); ++Idx) {
     int16_t SubIdx = BaseIndices[Idx];
-    Register Reg = RI.getSubReg(DestReg, SubIdx);
+    Register DestSubReg = RI.getSubReg(DestReg, SubIdx);
+    Register SrcSubReg = RI.getSubReg(SrcReg, SubIdx);
+    assert(DestSubReg && SrcSubReg && "Failed to find subregs!");
     unsigned Opcode = AMDGPU::S_MOV_B32;
 
     // Is SGPR aligned? If so try to combine with next.
-    Register Src = RI.getSubReg(SrcReg, SubIdx);
-    bool AlignedDest = ((Reg - AMDGPU::SGPR0) % 2) == 0;
-    bool AlignedSrc = ((Src - AMDGPU::SGPR0) % 2) == 0;
+    bool AlignedDest = ((DestSubReg - AMDGPU::SGPR0) % 2) == 0;
+    bool AlignedSrc = ((SrcSubReg - AMDGPU::SGPR0) % 2) == 0;
     if (AlignedDest && AlignedSrc && (Idx + 1 < BaseIndices.size())) {
       // Can use SGPR64 copy
       unsigned Channel = RI.getChannelFromSubReg(SubIdx);
       SubIdx = RI.getSubRegFromChannel(Channel, 2);
+      DestSubReg = RI.getSubReg(DestReg, SubIdx);
+      SrcSubReg = RI.getSubReg(SrcReg, SubIdx);
+      assert(DestSubReg && SrcSubReg && "Failed to find subregs!");
       Opcode = AMDGPU::S_MOV_B64;
       Idx++;
     }
 
-    LastMI = BuildMI(MBB, I, DL, TII.get(Opcode), RI.getSubReg(DestReg, SubIdx))
-                 .addReg(RI.getSubReg(SrcReg, SubIdx))
+    LastMI = BuildMI(MBB, I, DL, TII.get(Opcode), DestSubReg)
+                 .addReg(SrcSubReg)
                  .addReg(SrcReg, RegState::Implicit);
 
     if (!FirstMI)
@@ -1098,37 +1102,36 @@ void SIInstrInfo::copyPhysReg(MachineBasicBlock &MBB,
       SubIdx = SubIndices[Idx];
     else
       SubIdx = SubIndices[SubIndices.size() - Idx - 1];
+    Register DestSubReg = RI.getSubReg(DestReg, SubIdx);
+    Register SrcSubReg = RI.getSubReg(SrcReg, SubIdx);
+    assert(DestSubReg && SrcSubReg && "Failed to find subregs!");
 
     bool IsFirstSubreg = Idx == 0;
     bool UseKill = CanKillSuperReg && Idx == SubIndices.size() - 1;
 
     if (Opcode == AMDGPU::INSTRUCTION_LIST_END) {
       Register ImpDefSuper = IsFirstSubreg ? Register(DestReg) : Register();
       Register ImpUseSuper = SrcReg;
-      indirectCopyToAGPR(*this, MBB, MI, DL, RI.getSubReg(DestReg, SubIdx),
-                         RI.getSubReg(SrcReg, SubIdx), UseKill, *RS, Overlap,
-                         ImpDefSuper, ImpUseSuper);
+      indirectCopyToAGPR(*this, MBB, MI, DL, DestSubReg, SrcSubReg, UseKill,
+                         *RS, Overlap, ImpDefSuper, ImpUseSuper);
     } else if (Opcode == AMDGPU::V_PK_MOV_B32) {
-      Register DstSubReg = RI.getSubReg(DestReg, SubIdx);
-      Register SrcSubReg = RI.getSubReg(SrcReg, SubIdx);
       MachineInstrBuilder MIB =
-        BuildMI(MBB, MI, DL, get(AMDGPU::V_PK_MOV_B32), DstSubReg)
-        .addImm(SISrcMods::OP_SEL_1)
-        .addReg(SrcSubReg)
-        .addImm(SISrcMods::OP_SEL_0 | SISrcMods::OP_SEL_1)
-        .addReg(SrcSubReg)
-        .addImm(0) // op_sel_lo
-        .addImm(0) // op_sel_hi
-        .addImm(0) // neg_lo
-        .addImm(0) // neg_hi
-        .addImm(0) // clamp
-        .addReg(SrcReg, getKillRegState(UseKill) | RegState::Implicit);
+          BuildMI(MBB, MI, DL, get(AMDGPU::V_PK_MOV_B32), DestSubReg)
+              .addImm(SISrcMods::OP_SEL_1)
+              .addReg(SrcSubReg)
+              .addImm(SISrcMods::OP_SEL_0 | SISrcMods::OP_SEL_1)
+              .addReg(SrcSubReg)
+              .addImm(0) // op_sel_lo
+              .addImm(0) // op_sel_hi
+              .addImm(0) // neg_lo
+              .addImm(0) // neg_hi
+              .addImm(0) // clamp
+              .addReg(SrcReg, getKillRegState(UseKill) | RegState::Implicit);
       if (IsFirstSubreg)
         MIB.addReg(DestReg, RegState::Define | RegState::Implicit);
     } else {
       MachineInstrBuilder Builder =
-        BuildMI(MBB, MI, DL, get(Opcode), RI.getSubReg(DestReg, SubIdx))
-        .addReg(RI.getSubReg(SrcReg, SubIdx));
+          BuildMI(MBB, MI, DL, get(Opcode), DestSubReg).addReg(SrcSubReg);
       if (IsFirstSubreg)
         Builder.addReg(DestReg, RegState::Define | RegState::Implicit);