[SDAG] Refine MMO size when converting masked load/store to normal load/store

After D113888 / 32b6c17b the MMO size of a masked loads/store is
unknown. When we are converting back to a standard load/store because
the mask is known all ones, we can refine that to the correct size from
the size of the vector being loaded/stored.

Differential Revision: https://reviews.llvm.org/D114582