[clang] [clang] Implement __builtin_{clzg,ctzg} (PR #83431)

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@@ -3157,7 +3177,15 @@ RValue CodeGenFunction::EmitBuiltinExpr(const GlobalDecl GD, unsigned BuiltinID,
     if (Result->getType() != ResultType)
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overmighty wrote:

Since in the additions below I check if the argument itself is zero instead of checking the result of `ctlz`, I don't see why the `ctlz` result for zero being undefined would be a problem.

https://github.com/llvm/llvm-project/pull/83431