[RISCV] Use TU vmv.v.v instead of vslideup.vi N,M,0 for insert vector_shuffles

This applies the same technique in https://reviews.llvm.org/D152565 to
vslideups when lowering vector_shuffles that have an insert pattern in
their mask.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D152724