[RISCV] Custom isel the rest of the vector load/store intrinsics.

A previous patch moved the index versions. This moves the rest.
I also removed the custom lowering for VLEFF since we can now
do everything directly in the isel handling.

I had to update getLMUL to handle mask registers to index the
pseudo table correctly for VLE1/VSE1.

This is good for another 15K reduction in llc size.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D97097