[llvm] [AMDGPU][RFC] Combine asm and disasm tests. (PR #90214)

[Fangrui Song via llvm-commits]

MaskRay wrote:

#92895, which does not add another MCStreamer, looks fine to me.
Note, we really should not add another MCStreamer, which is very easy to be misused.

MCAsmStreamer has some existing issues related to expression evaluation. If another MCStreamer is added, I am concerned others might use it for more assembler tasks (e.g. to support `.section`), which make future maintenance more difficult.
If the llvm-objdump output isn't convenient for AMDGPU, I think it is fine to add a new output mode.

https://github.com/llvm/llvm-project/pull/90214