[llvm] [SelectionDAG] Avoid illegal wide vectors expanding get.active.lane.mask (PR #213670)

[Luke Lau via llvm-commits]

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@@ -505,6 +505,15 @@ class LLVM_ABI TargetLoweringBase {
     return true;
   }
 
+  /// Return the vector type that the generic expansion of
+  /// @llvm.get.active.lane.mask in SelectionDAGBuilder builds its step and
+  /// splat vectors at, for a mask of type \p ResVT and scalar operands of type
+  /// \p OpVT. This is a vector of \p OpVT, except when that vector is a
+  /// scalable vector the type legalizer would have to scalarize, in which case
+  /// the expansion falls back to a narrower element type.
+  EVT getGetActiveLaneMaskExpansionVT(LLVMContext &Context, EVT ResVT,
+                                      EVT OpVT) const;
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lukel97 wrote:

Can we reuse `getBitWidthForCttzElements` instead of adding a new hook? Its used for a similar purpose for expanding cttz.elts etc.

https://github.com/llvm/llvm-project/pull/213670