[llvm] GlobalISel needs fdiv 1 / sqrt(x) to rsq combine (PR #78673)

[Thorsten Schütt via llvm-commits]

================
@@ -334,6 +337,59 @@ bool AMDGPUPostLegalizerCombinerImpl::matchRcpSqrtToRsq(
   return false;
 }
 
+bool AMDGPUPostLegalizerCombinerImpl::matchFDivSqrt(
+    MachineInstr &MI,
+    std::function<void(MachineIRBuilder &)> &MatchInfo) const {
+
+  // TODO: Can I match fdiv 1.0 / sqrt(x) from here?
+  // My apologies, this code is still a mess. Trying to figure out
+  // what value MI should hold when getting to this point
+
+  auto getSqrtSrc = [=](const MachineInstr &MI) -> MachineInstr * {
----------------
tschuett wrote:

Does not compile, but ...
```
std::optional<FPValueAndVReg > One;
Register SqrtReg;
if (!mi_match(MI.getOperand(1).getReg(), MRI, m_GFCstOrSplat(One)) || !mi_match(MI.getOperand(2).getReg(), MIR, m_OneNonDBGUse (m_GFSqrt(m_Reg(SqrtReg)))
  return false;
```
```

https://github.com/llvm/llvm-project/pull/78673