[RISCV] Teach doPeepholeMaskedRVV to handle vslide{up,down}

We already handle vslide1{up,down}, so this extends it to vslide{up,down}.

This was unintentionally added in https://reviews.llvm.org/D150463 and
then removed in 37cfcfce, but unless I'm
missing something this should still be ok as the mask only controls what
destination elements are written to.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D153631