[RISCV] Add tests covering shuffles which can be optimized

These shuffles all take the form of a "splat" of the LHS and/or RHS to
some degree, with one or two elements needing patched up afterwards. We
currently lower all of these to full LHS/RHS vector-index shuffles with
vrgather.vv.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D107447