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

[Matt Arsenault via llvm-commits]

================
@@ -7622,10 +7624,14 @@ LegalizerHelper::lowerSADDO_SSUBO(MachineInstr &MI) {
   LLT Ty = Dst0Ty;
   LLT BoolTy = Dst1Ty;
 
+  auto Tmp = MRI.createGenericVirtualRegister(MRI.getType(Dst0));
+
   if (IsAdd)
-    MIRBuilder.buildAdd(Dst0, LHS, RHS);
+    MIRBuilder.buildAdd(Tmp, LHS, RHS);
   else
-    MIRBuilder.buildSub(Dst0, LHS, RHS);
+    MIRBuilder.buildSub(Tmp, LHS, RHS);
+
+  MIRBuilder.buildCopy(Dst0, Tmp);
----------------
arsenm wrote:

auto Tmp = IsAdd ? buildAdd() : buildSub 

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