[RISCV] Add tests for vnsr[l,a].wx patterns that could be matched

These patterns of ([l,a]shr v, ([s,z]ext splat)) only pick up the cases where
the scalar has the same type as the vector element. However since only the low
log2(SEW) bits of the scalar are read, we could use any scalar type that has
been extended.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D155697

Author: lukel97