[SelectionDAG] Handle VSCALE in isKnownNeverZero

[lukel97]

VSCALE is by definition greater than zero, but this checks it via getVScaleRange anyway.

The motivation for this is to be able to check if the EVL for a VP strided load is non-zero in #97394.

I added the tests to the RISC-V backend since the existing X86 known-never-zero.ll test crashed when trying to lower vscale for the +sse2 RUN line.