[RISCV] Add basic support for matching shuffles to vslidedown.vi.

Specifically the unary shuffle case where the elements being
shifted in are undef. This handles the shuffles produce by expanding
llvm.reduce.mul.

I did not reduce the VL which would increase the number of vsetvlis,
but may improve the execution speed. We'd also want to narrow the
multiplies so we could share vsetvlis between the vslidedown.vi and
the next multiply.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D117239