[llvm] [ConstantFolding] Fold scalable get_active_lane_masks (PR #156659)

[Matthew Devereau via llvm-commits]

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@@ -4238,6 +4238,13 @@ static Constant *ConstantFoldScalableVectorCall(
 
     return ConstantInt::getFalse(SVTy);
   }
+  case Intrinsic::get_active_lane_mask: {
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MDevereau wrote:

As it stands, ConstantFolding can already handle this for fixed-width. Scalable seems to have been left behind: https://godbolt.org/z/snd7M5oer 

https://github.com/llvm/llvm-project/pull/156659