[llvm] [AMDGPU] Only try DecoderTables for the current subtarget. NFCI. (PR #82992)

[Jay Foad via llvm-commits]

jayfoad wrote:

> We would not need to do the checks manually if the disassembler TableGen backend a) tests predicates as early as possible and b) allows differently predicated / mutually exclusive instructions in the same decoding table.

If `DisassemblerEmitter` worked like that then I don't think we would need to use different `DecoderNamespace`s at all.

https://github.com/llvm/llvm-project/pull/82992