AMDGPU: Relax legal register operand constraint (#157989)
Find a common subclass instead of directly checking for a subclass
relationship. This fixes folding logic for unaligned register defs
into aligned use contexts. e.g., a vreg_64 def into an av_64_align2
use should be able to find the common subclass vreg_align2. This
avoids regressions in future patches.
Checking the subclass was also redundant on the subregister path;
getMatchingSuperRegClass is sufficient.
