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

[Pierre van Houtryve via llvm-commits]

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@@ -33,6 +33,11 @@ def rcp_sqrt_to_rsq : GICombineRule<
          [{ return matchRcpSqrtToRsq(*${rcp}, ${matchinfo}); }]),
   (apply [{ Helper.applyBuildFn(*${rcp}, ${matchinfo}); }])>;
 
+def fdiv_1_by_sqrt_to_rsq : GICombineRule<
+  (defs root:$root, build_fn_matchinfo:$matchinfo),
+  (match (wip_match_opcode G_FSQRT, G_FDIV):$root,
+         [{ return matchFDivSqrt(*${root}, ${matchinfo}); }]),
----------------
Pierre-vh wrote:

You can do this fully in MIR patterns :) https://llvm.org/docs/GlobalISel/MIRPatterns.html
We just can't match FP constants yet so that needs to be done in C++

I also don't know if we want this for half only or for all FP types. If it's the former, you'd also  need to check that the type of `$dst` is `LLT::scalar(16)`. 
```suggestion
  (defs root:$dst),
  (match (G_FSQRT $sqrt, $x),
         (G_FCONSTANT $one, $fpimm),
         (G_FDIV $dst, $sqrt, $fpimm, (MIFlags FmContract)),
         [{ return ${fpimm}.getOperand(1).isExactlyValue(1.0); }]),
```

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