[clang] [clang] Introduce elementwise clz/ctz builtins (PR #131995)

[Matt Arsenault via cfe-commits]

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@@ -831,6 +832,14 @@ of different sizes and signs is forbidden in binary and ternary builtins.
                                                 semantics, see `LangRef
                                                 <http://llvm.org/docs/LangRef.html#llvm-min-intrinsics-comparation>`_
                                                 for the comparison.
+ T __builtin_elementwise_clz(T x[, T y])        return the number of leading 0 bits in the first argument. If          integer types
+                                                the first argument is 0 and an optional second argument is provided,
+                                                the second argument is returned. If the first argument is 0 but only
+                                                one argument is provided, the behaviour is undefined.
+ T __builtin_elementwise_ctz(T x[, T y])        return the number of trailing 0 bits in the first argument. If         integer types
+                                                the first argument is 0 and an optional second argument is provided,
+                                                the second argument is returned. If the first argument is 0 but only
+                                                one argument is provided, the behaviour is undefined.
----------------
arsenm wrote:

On regular __builtin_clz/ctz the gcc docs state "result is undefined". Not the program is undefined 

https://github.com/llvm/llvm-project/pull/131995