[RISCV] Add a combine to form masked.load from unit strided load (#65674)

Add a DAG combine to form a masked.load from a masked_strided_load
intrinsic with stride equal to element size. This covers a couple of
extra test cases, and allows us to simplify and common some existing
code on the concat_vector(load, ...) to strided load transform.

This is the first in a mini-patch series to try and generalize our
strided load and gather matching to handle more cases, and common up
different approaches to the same problems in different places.