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

[Pierre van Houtryve via llvm-commits]

================
@@ -334,6 +336,19 @@ bool AMDGPUPostLegalizerCombinerImpl::matchRcpSqrtToRsq(
   return false;
 }
 
+void AMDGPUPostLegalizerCombinerImpl::applyOneFDivSqrtToRsq(
+    MachineInstr &MI, const Register &X) const {
+  // B.setInstrAndDebugLoc(MI);
+
+  Register Dst = MI.getOperand(0).getReg();
+
+  B.buildIntrinsic(Intrinsic::amdgcn_rsq, ArrayRef<Register>({Dst}))
+      .addUse(X)
+      .setMIFlags(MI.getFlags());
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Pierre-vh wrote:

I'm not sure at all if that's needed (or if it's even correct - could we contract two operations again after applying contract already?)
cc @arsenm / @jayfoad 

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