[llvm] [SCEV] Simplify SCEVExpr for PHI to SCEV for operand if operands are identical (PR #115945)

[Nikita Popov via llvm-commits]

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@@ -6019,6 +6019,34 @@ const SCEV *ScalarEvolution::createNodeFromSelectLikePHI(PHINode *PN) {
   return nullptr;
 }
 
+/// Returns SCEV for the first operand of a phi if all phi operands have
+/// identical opcodes and operands
+/// eg.
+/// a: %add = %a + %b
+///    br %c
+/// b: %add1 = %a + %b
+///    br %c
+/// c: %phi = phi [%add, a], [%add1, b]
+/// scev(%phi) => scev(%add)
+const SCEV *
+ScalarEvolution::createNodeForPHIWithIdenticalOperands(PHINode *PN) {
+  BinaryOperator *CommonInst = nullptr;
+  for (Value *Incoming : PN->incoming_values()) {
+    BinaryOperator *IncomingInst = dyn_cast<BinaryOperator>(Incoming);
+    if (CommonInst) {
+      if (!(IncomingInst && CommonInst->isIdenticalTo(IncomingInst)))
----------------
nikic wrote:

```suggestion
      if (!(IncomingInst && CommonInst->isIdenticalToWhenDefined(IncomingInst)))
```
I think that's what you want? Your motivating example has different nowrap flags on the two adds, so isIdentical() wouldn't help for it.

https://github.com/llvm/llvm-project/pull/115945