[RISCV] Use tail undisturbed vmv.v.v instead of vslideup.vi vN, vM, 0 for subvector insertion

vslideup has a vector overlap constraint that vmv.v.v doesn't.
vmv.v.v is also a simpler instruction so may have better
throughput and/or latency in some CPUs.

This is an alternative to D152298, D152368, and D152496.

Reviewed By: luke, reames

Differential Revision: https://reviews.llvm.org/D152565