[GlobalISel] Look through extends etc in CombinerHelper::matchConstantOp

It's possible to end up with a zext or something in the way of a G_CONSTANT,
even pre-legalization. This can happen with memsets.

e.g.

https://godbolt.org/z/Bjc8cw

To make sure we can catch these cases, use `getConstantVRegValWithLookThrough`
instead of `mi_match`.

Differential Revision: https://reviews.llvm.org/D81875

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

@@ -1617,8 +1617,9 @@ bool CombinerHelper::matchEqualDefs(const MachineOperand &MOP1,
 bool CombinerHelper::matchConstantOp(const MachineOperand &MOP, int64_t C) {
   if (!MOP.isReg())
     return false;
-  int64_t Cst;
-  return mi_match(MOP.getReg(), MRI, m_ICst(Cst)) && Cst == C;
+  // MIPatternMatch doesn't let us look through G_ZEXT etc.
+  auto ValAndVReg = getConstantVRegValWithLookThrough(MOP.getReg(), MRI);
+  return ValAndVReg && ValAndVReg->Value == C;
 }
 
 bool CombinerHelper::replaceSingleDefInstWithOperand(MachineInstr &MI,

llvm/test/CodeGen/AArch64/GlobalISel/prelegalizercombiner-trivial-arith.mir

@@ -286,4 +286,24 @@ body:             |
     %op:_(s32) = G_SUB %x(s32), %cst
     $w0 = COPY %op(s32)
     RET_ReallyLR implicit $w0
+
+...
+---
+name:            look_through_zext
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $x0
+    ; CHECK-LABEL: name: look_through_zext
+    ; CHECK: liveins: $x0
+    ; CHECK: %zero:_(s8) = G_CONSTANT i8 0
+    ; CHECK: %zext_zero:_(s64) = G_ZEXT %zero(s8)
+    ; CHECK: $x0 = COPY %zext_zero(s64)
+    ; CHECK: RET_ReallyLR implicit $x0
+    %zero:_(s8) = G_CONSTANT i8 0
+    %zext_zero:_(s64) = G_ZEXT %zero(s8)
+    %c:_(s64) = G_CONSTANT i64 72340172838076673
+    %mul:_(s64) = G_MUL %c, %zext_zero
+    $x0 = COPY %mul(s64)
+    RET_ReallyLR implicit $x0
 ...