[llvm] [InstCombine][Docs] Update InstCombine contributor guide (PR #144228)

[Ramkumar Ramachandra via llvm-commits]

================
@@ -531,6 +551,19 @@ need to add a one-use check for the inner instruction.
 One-use checks can be performed using the `m_OneUse()` matcher, or the
 `V->hasOneUse()` method.
 
+### Flag handling
+
+When possible, propagate poison-generating flags like `nuw` and `nsw` since they may be
+hard to salvage later. If it is not free (e.g. requires additional complexity like `willNotOverflow`
+or KnownBits queries), don't do that.
+
+Be careful with in-place operand/predicate changes, as poison-generating flags may not be valid for new
+operands. It is recommended to create a new instruction with carefully handling of flags. If not
+applicable, call `Instruction::dropPoisonGeneratingFlags()` to clear flags in a conservative manner.
+
+Do not rely on fcmp's `nsz` flag to perform optimizations. It is meaningless for fcmp so it should not affect
+the optimization.
----------------
artagnon wrote:

Perhaps I'm confused about something? See CmpInst::isEquivalence, which is used in some places:

```cpp
// Returns true if either operand of CmpInst is a provably non-zero
// floating-point constant.
static bool hasNonZeroFPOperands(const CmpInst *Cmp) {
  auto *LHS = dyn_cast<Constant>(Cmp->getOperand(0));
  auto *RHS = dyn_cast<Constant>(Cmp->getOperand(1));
  if (auto *Const = LHS ? LHS : RHS) {
    using namespace llvm::PatternMatch;
    return match(Const, m_NonZeroNotDenormalFP());
  }
  return false;
}

// Floating-point equality is not an equivalence when comparing +0.0 with
// -0.0, when comparing NaN with another value, or when flushing
// denormals-to-zero.
bool CmpInst::isEquivalence(bool Invert) const {
  switch (Invert ? getInversePredicate() : getPredicate()) {
  case CmpInst::Predicate::ICMP_EQ:
    return true;
  case CmpInst::Predicate::FCMP_UEQ:
    if (!hasNoNaNs())
      return false;
    [[fallthrough]];
  case CmpInst::Predicate::FCMP_OEQ:
    return hasNonZeroFPOperands(this);
  default:
    return false;
  }
}
```

https://github.com/llvm/llvm-project/pull/144228