[llvm] [InstCombinePHI] Enhance PHI CSE to remove redundant phis (PR #163453)

[Yingwei Zheng via llvm-commits]

https://github.com/dtcxzyw commented:

I find that it is more natural to handle this pattern in `simplifySelectInst`:
```
bool isSimplifierIdenticalPHI(PHINode &PHI, PHINode &IdenticalPHI) {
  if (PHI.getParent() != IdenticalPHI.getParent())
    return false;
  // Check incoming values
  ...
  // Check next values
  ...
}
Value *simplifySelectInst(Value *Cond, Value *TrueVal, Value *FalseVal,
                                 const SimplifyQuery &Q, unsigned MaxRecurse) {
  ...

  if (auto *TruePHI = dyn_cast<PHINode>(TrueVal)) {
    if (auto *FalsePHI = dyn_cast<PHINode>(FalseVal)) {
    if (isSimplifierIdenticalPHI(*TruePHI, *FalsePHI))
      return FalseVal;
    if (isSimplifierIdenticalPHI(*TruePHI, *FalsePHI))
      return FalseVal;
    if (isSimplifierIdenticalPHI(*FalsePHI, *TruePHI))
      return TrueVal;
  }
  }
  return nullptr;
}
```


https://github.com/llvm/llvm-project/pull/163453