[llvm] [AMDGPU][True16][CodeGen] fix moveToVALU with proper subreg access in true16 (PR #132089)

[Brox Chen via llvm-commits]

broxigarchen wrote:

> There was a comment on the original PR #131859 from @arsenm that 'you should be able to get this correct from the start'. I don't think we can get it right from the start, because the core issue is putting both 16 and 32 bit values into SGPR32.
> 
> So this patch and #131859 are different ways of fixing that up. This patch is better in one way because it attaches proper subregisters, where the other one doesn't. But this patch adds more special purpose code.
> 
> I see we don't have exiting uses of SUBREG_TO_REG in the AMDGPU backend. Where is that lowered to a target instruction? Do we expect passes seeing it to understand it? If so, this patch seems ok to me.

Hi Joe. SUBREG_TO_REG is lowered to copy in a codegen pass https://github.com/llvm/llvm-project/blob/main/llvm/lib/CodeGen/ExpandPostRAPseudos.cpp

https://github.com/llvm/llvm-project/pull/132089