[clang] [llvm] AMDGPU: Add builtin/intrinsic global_(load|store)_b128 (PR #172090)

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macurtis-amd wrote:

@shiltian @arsenm:
Are you ok with @ssahasra's suggestion:
>the Clang builtins should take a standard scope enum just like the scoped atomics, but the LLVM intrinsic can take a string literal and parse it in the same way as other scoped intrinsics.

https://github.com/llvm/llvm-project/pull/172090