[llvm] [AMDGPU] Add amdgpu-accum-offset attribute to bound available arch VGPR (PR #218745)

[via llvm-commits]

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JoshuaGrindstaff wrote:

It's not an error, it's possible for amdgpu-accum-offset to be set pessimistically for being external (so it doesn't print an attribute) while inheriting the amdgpu-agpr-alloc attribute from one of it's callees. 

The codegen will work fine with either attribute missing. Missing accum-offset will set the VGPR max to `NumRegs/2` and a missing agpr-alloc will set the AGPR max to `NumRegs/2` and AGPR min to 0.

https://github.com/llvm/llvm-project/pull/218745