[AMDGPU] Allow SOP asm mnemonic to differ

Allows the creation of real SOP1 instructions with
assembler mnemonics that differ from their
pseudo-instruction mnemonics. The default behavior
keeps the mnemonics matching.

Corrects a subtarget label typo in a comment.

Authored By: Joe_Nash

Differential Revision: https://reviews.llvm.org/D88708