[MachineCopyPropagation] When the source of PreviousCopy is undef, we cannot replace sub register

The `postrapseudos` may replace `mov w8, w0` with `mov x8, x0`.

Author: dianqk

llvm/lib/CodeGen/MachineCopyPropagation.cpp

@@ -443,6 +443,9 @@ static bool isNopCopy(const MachineInstr &PreviousCopy, MCRegister Src,
     return true;
   if (!TRI->isSubRegister(PreviousSrc, Src))
     return false;
+  // When the source of PreviousCopy is undef, we cannot replace sub register.
+  if (CopyOperands->Source->isUndef())
+    return false;
   unsigned SubIdx = TRI->getSubRegIndex(PreviousSrc, Src);
   return SubIdx == TRI->getSubRegIndex(PreviousDef, Def);
 }

llvm/test/CodeGen/AArch64/machine-cp-undef.mir

@@ -10,7 +10,8 @@ body:             |
   ; CHECK-NEXT:   successors: %bb.1(0x80000000)
   ; CHECK-NEXT:   liveins: $w0
   ; CHECK-NEXT: {{  $}}
-  ; CHECK-NEXT:   $x8 = ORRXrs $xzr, $x0, 0, implicit $w0
+  ; CHECK-NEXT:   $x8 = ORRXrs $xzr, undef $x0, 0, implicit $w0
+  ; CHECK-NEXT:   $w8 = ORRWrs $wzr, $w0, 0, implicit-def $x8
   ; CHECK-NEXT: {{  $}}
   ; CHECK-NEXT: bb.1:
   ; CHECK-NEXT:   liveins: $x8