[RISCV] Use v(f)slide1up for shuffle+insert idiom

[RISCV] Use v(f)slide1up for shuffle+insert idiom

This is pretty straight forward in the basic form. I did need to move the slideup matching earlier, but that looks generally profitable on it's own.

As follow ups, I plan to explore the v(f)slide1down variants, and see what I can do to canonicalize the shuffle then insert pattern (see _inverse tests at the end of the vslide1up.ll test).

Differential Revision: https://reviews.llvm.org/D151468