[llvm] [AMDGPU] Canonicalize G_ZEXT of the shift amount in RegBankCombiner (PR #131792)

[Pierre van Houtryve via llvm-commits]

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@@ -134,6 +134,22 @@ def combine_fmul_with_select_to_fldexp : GICombineRule<
           [{ return Helper.matchCombineFmulWithSelectToFldexp(*${root}, *${sel}, ${matchinfo}); }]),
   (apply  [{ Helper.applyBuildFn(*${root}, ${matchinfo}); }])>;
 
+// (shift x, (zext amt)) -> (shift x, (and (anyext amt), mask)
+//
+// The pattern is longer, but is better for matching during ISel.
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Pierre-vh wrote:

Yeah ideally ISel should just handle more cases, but I wasted a lot of hours trying to make the ISel patterns work but I lack experience in that area so I couldn't get it done, I always got weird inference errors

https://github.com/llvm/llvm-project/pull/131792