[RISCV] Use v(f)slide1down for shuffle+insert idiom

This is a follow up to D151468 which added the vslide1down case as a sub-case of vslide1down matching. This generalizes that code into generic mask matching - specifically to point out the sub-vector insert restriction in the original patch. Since the matching logic is basically the same, go ahead and support vslide1up at the same time.

Differential Revision: https://reviews.llvm.org/D151742