[llvm] [AMDGPU] Add missing tests for GFX10 (t)buffer format d16 instructions (PR #84789)

[Jay Foad via llvm-commits]

jayfoad wrote:

For GFX8 and GFX9 these instructions were tested in:
```
test/MC/AMDGPU/buf-fmt-d16-packed.s
test/MC/AMDGPU/buf-fmt-d16-unpacked.s
test/MC/Disassembler/AMDGPU/buf_fmt_packed_d16.txt
test/MC/Disassembler/AMDGPU/buf_fmt_unpacked_d16.txt
```
These files were not updated for GFX10.

https://github.com/llvm/llvm-project/pull/84789