[llvm] [InstCombine] Optimise x / sqrt(y / z) with fast-math pattern. (PR #76737)

[Zain Jaffal via llvm-commits]

================
@@ -1701,6 +1701,31 @@ static Instruction *foldFDivPowDivisor(BinaryOperator &I,
   return BinaryOperator::CreateFMulFMF(Op0, Pow, &I);
 }
 
+/// Convert div to mul if we have an sqrt divisor iff sqrt's operand is a fdiv
+/// instruction.
+static Instruction *foldFDivSqrtDivisor(BinaryOperator &I,
+                                        InstCombiner::BuilderTy &Builder) {
+  // X / sqrt(Y / Z) -->  X * sqrt(Z / Y)
+  Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
+  auto *II = dyn_cast<IntrinsicInst>(Op1);
+  if (!II || II->getIntrinsicID() != Intrinsic::sqrt || !II->hasOneUse() ||
+      !I.hasAllowReassoc() || !I.hasAllowReciprocal())
----------------
zjaffal wrote:

okay so from my understanding then the pattern we are looking for is the following 

```
%div = fdiv arcp double %y, %z
%sqrt = call reassoc double @llvm.sqrt.f64(double %div)
%div2 = fdiv arcp double %x, %sqrt
```


https://github.com/llvm/llvm-project/pull/76737