[AMDGPU][AsmParser] Eliminate cvtMtbuf().

Now that we have proper support for optional operands, the standard LLVM
machinery can take care of converting parsed instructions to MCInsts.
There are likely more cases where the conversion can be done
automatically, probably with some additional treatment. The plan is to
address them separately.

Part of <https://github.com/llvm/llvm-project/issues/62629>.

Reviewed By: arsenm, foad

Differential Revision: https://reviews.llvm.org/D153565