[llvm] [SelectionDAG] Avoid irregular INSERT_SUBVECTOR when widening CTTZ_ELTS (PR #218019)

[Simon Pilgrim via llvm-commits]

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@@ -101,5 +101,24 @@ define i8 @ctz_v8i16_poison(<8 x i16> %a) {
   ret i8 %res
 }
 
+; Irregular predicate vectors widen to a power-of-two number of lanes. The
+; widened lanes must be active so an all-zero input returns the original lane
+; count, without requiring an irregular INSERT_SUBVECTOR during legalization.
+define i32 @ctz_zero_v17i1() {
+; CHECK-LABEL: ctz_zero_v17i1:
+; CHECK:         retq
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RKSimon wrote:

nocodegen at all?

https://github.com/llvm/llvm-project/pull/218019