[RISCV] Use SplatPat/SplatPat_simm5 to handle PseudoVMV_V_X_/PseudoVMV_V_I_ selection as well.

This ensures that we'll match immediates consistently regardless
of whether we match them as a standalone splat or as part of
another operation.

While I was there I added complexities to the simm5/uimm5 patterns so
we didn't have to assume that the 1 on the non-immediate was lower
than what tablegen inferred.

I had to make a minor tweak to tablegen to fix one place that
didn't expect to see a ComplexPattern that wasn't a "leaf".

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D96199