[MIPS GlobalISel] Select 4 byte unaligned load and store

Improve legality checks for load and store, 4 byte scalar
load and store are now legal for all subtargets.
During regbank selection 4 byte unaligned loads and stores
for MIPS32r5 and older get mapped to gprb.
Select 4 byte unaligned loads and stores for MIPS32r5.
Fix tests that unintentionally had unaligned load or store.

Differential Revision: https://reviews.llvm.org/D74624