[RISCV] Refactor selectVSplat. NFCI

This patch shares the logic between the various splat ComplexPatterns to help
the diff in some upcoming patches.

It's worth noting that the uimm splat pattern now takes into account the
implicit truncation + sign extend semantics of vmv_v_x_vl, but that doesn't
seem to affect the result since it always took the sext value anyway.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D158741