[llvm] [GlobalISel] Fold G_ICMP if possible (PR #86357)

[Amara Emerson via llvm-commits]

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@@ -7638,7 +7644,7 @@ LegalizerHelper::lowerSADDO_SSUBO(MachineInstr &MI) {
   // (LHS) if and only if the other operand (RHS) is (non-zero) positive,
   // otherwise there will be overflow.
   auto ResultLowerThanLHS =
-      MIRBuilder.buildICmp(CmpInst::ICMP_SLT, BoolTy, Dst0, LHS);
+      MIRBuilder.buildICmp(CmpInst::ICMP_SLT, BoolTy, Tmp, LHS);
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aemerson wrote:

@shiltian  What if we assert in `buildInstr()` that `DstOp` vregs are undefined? That will then force clients to `eraseFromParent()` *before* calling it.

The other idea of having `buildInstr()` erase the MI itself forces each vreg build to call `MRI.getVRegDef()` even if it's never needed, and it's probably unintuitive from an API perspective.

https://github.com/llvm/llvm-project/pull/86357