[RISCV] Prefer vnsrl.wi v8, v8, 0 over vnsrl.wx v8, v8, x0.

I have a couple data points that some microarchitectures prefer
the immediate 0 over x0. Does anyone know of microarchitectures
where the opposite is true?

Unfortunately, this is different than the vncvt.x.x.w alias
from the spec. Perhaps the alias was poorly chosen if x0 isn't
as optimal as immediate 0 on all microarchitectures.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D132041