[AMDGPU] Set predicates more consistently for BUF instructions (#81865)

Set DecoderNamespace and AssemblerPredicate in the base class for Real
instructions for each subtarget. This avoids some ad hoc "let" around
groups of instructions definitions, and fixes some missed cases like
BUFFER_GL0_INV_gfx10 which was missing DecoderNamespace.

Author: jayfoad