[AMDGPU][MC] Added flag to identify VOP instructions which have a single variant

By convention, VOP1/2/C instructions which can be promoted to VOP3 have _e32 suffix while promoted instructions have _e64 suffix. Instructions which have a single variant should have no _e32/_e64 suffix. Unfortunately there was no simple way to identify single variant instructions - it was implemented by a hack. See bug https://bugs.llvm.org/show_bug.cgi?id=39086.

This fix simplifies handling of single VOP instructions by adding a dedicated flag.

Differential Revision: https://reviews.llvm.org/D99408