[llvm] [CodeGenPrepare][RISCV] Combine (X ^ Y) and (X == Y) where appropriate (PR #130922)

[Ryan Buchner via llvm-commits]

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@@ -17194,12 +17194,56 @@ static bool combine_CC(SDValue &LHS, SDValue &RHS, SDValue &CC, const SDLoc &DL,
     return true;
   }
 
+  // If XOR is reused and has an immediate that will fit in XORI,
+  // do not fold.
+  auto IsXorImmediate = [](const SDValue &Op) -> bool {
+    if (const auto XorCnst = dyn_cast<ConstantSDNode>(Op))
+      return isInt<12>(XorCnst->getSExtValue());
+    return false;
+  };
+  // Fold (X(i1) ^ 1) == 0 -> X != 0
+  auto SingleBitOp = [&DAG](const SDValue &VarOp,
+                            const SDValue &ConstOp) -> bool {
+    if (const auto XorCnst = dyn_cast<ConstantSDNode>(ConstOp)) {
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bababuck wrote:

My understanding is that this would require a `try-catch` statement since the failing case of `dyn_cast<>()` when returning a reference is to throw a `std::bad_cast`. Because of that I am thinking that returning a pointer will result in cleaner code, but if there is a strong reason for returning a reference please let me know.

I will make the change to explicitly declare that `XorCnst` is a pointer type (as suggested here as well as in topperc's comment.

https://github.com/llvm/llvm-project/pull/130922