[clang] [llvm] [clang][AArch64] Omit zext when high bits have unspecified value (PR #214788)

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eleviant wrote:

> Instead of introducing an attribute like this, can we just change how we pass the value? For example, if we have a structure that's one byte, can we just pass it as i8?
> 
> The "bitwidth" attribute is a lot more complicated than this patch makes it look, particularly the interactions with interprocedural optimizations.

Makes sense. Closing in favor of https://github.com/llvm/llvm-project/pull/215264

https://github.com/llvm/llvm-project/pull/214788