[llvm] [SelectionDAG] Add CTTZ_ELTS[_ZERO_POISON] nodes. NFCI (PR #185600)

[Simon Pilgrim via llvm-commits]

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@@ -1577,6 +1577,11 @@ enum NodeType {
   /// Output: Output Chain
   EXPERIMENTAL_VECTOR_HISTOGRAM,
 
+  /// The `llvm.experimental.cttz.elts` intrinsic. Has a single i1 vector
+  /// operand.
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RKSimon wrote:

Instead of referring to the intrinsic, explicit describe the op - "Counts the number of trailing zero elements of a vector". Do we guarantee that the return type is large enough to cover all counts (i.e. i8 CTTZ_ELTS( v1024i1) is illegal?) - if so mention that.

https://github.com/llvm/llvm-project/pull/185600