[llvm] [X86] Implement canceling out of XOR with equality (PR #155106)

[Matt Arsenault via llvm-commits]

================
@@ -49009,6 +49009,28 @@ static SDValue combineSetCCMOVMSK(SDValue EFLAGS, X86::CondCode &CC,
 static SDValue combineSetCCEFLAGS(SDValue EFLAGS, X86::CondCode &CC,
                                   SelectionDAG &DAG,
                                   const X86Subtarget &Subtarget) {
+  // For EQ/NE, rewrite SUB(xor A,B, xor C,B) -> SUB(A,C) and
+  //            SUB(add A,K, add C,K)       -> SUB(A,C)
+  // Only safe if the arithmetic result is unused, since we only
+  // care about flags for EQ/NE.
+
+  // TODO: Can we do this for other comparions like 31 ^ A > 31 ^ B ?
+  if ((CC == X86::COND_E || CC == X86::COND_NE) &&
+      EFLAGS.getOpcode() == X86ISD::SUB &&
+      !EFLAGS.getNode()->hasAnyUseOfValue(0)) {
----------------
arsenm wrote:

```suggestion
      !EFLAGS.hasAnyUseOfValue(0)) {
```

https://github.com/llvm/llvm-project/pull/155106