[AMDGPU] Don't bother to use OffsetMode to define Real SMEM instructions

Various Real classes took an OffsetMode parameter, but only used it
to extract the suffix for the name of the corresponding pseudo. I found
this confusing because you couldn't usefully define and use a different
OffsetMode here, e.g. one with different operand types to affect how the
instruction was printed.

Overall I think it's simpler to just pass in the suffixed pseudo name
directly.

Differential Revision: https://reviews.llvm.org/D147242