[RISCV] Consolidate legality checking for strided load/store [nfc]

Note that the strided load from concat_vector combine was using the wrong legality test. It happened to work out as the alignment requirement is based on the scalar type either way, but unless I'm missing something allowsMisalignedAccess is expecting a contiguous memory access.

Differential Revision: https://reviews.llvm.org/D149369