[llvm] [AMDGPU][True16][CodeGen] Avoid setting hi part in copysign (PR #160891)

[Brox Chen via llvm-commits]

broxigarchen wrote:

Thanks for the fix! 

Looks like there is a missing case "%87:sreg_32 = COPY %86.hi16:vgpr_32" to be handled in si-fix-sgpr-copies.  I will create a seperate patch

https://github.com/llvm/llvm-project/pull/160891