[llvm] [WIP] [AMDGPU] [GlobalIsel] Combine Fmul with Select into ldexp instruction. (PR #120104)

[Matt Arsenault via llvm-commits]

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@@ -465,13 +465,13 @@ bool AMDGPUCombinerHelper::matchCombineFmulWithSelectToLdexp(
 
     const auto SelectTrueCst =
         DestTy.isVector()
-            ? getFConstantSplat(SelectTrue, MRI, /* allowUndef */ true)
+            ? getFConstantSplat(SelectTrue, MRI, /*allowUndef=*/false)
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arsenm wrote:

Please introduce the helper for dealing with splat or constants or drop vector support entirely 

https://github.com/llvm/llvm-project/pull/120104