[RISCV] Form vmv.s.f/x from single element splats via DAG combine

This re-implements the special casing we had in lowerScalarSplat as a DAG combine. As can be seen in the tests, this ends up triggering in a bunch more cases.

The semantically interesting bit of this change is the use of the implicit truncate semantics for when XLEN > SEW. We'd already been doing this for vmv.v.x, but this change extends e.g. the constant matching to make the same assumption about vmv.s.x. Per my reading of the specification, this should be fine, and if anything, is more obviously true of vmv.s.x than vmv.v.x.

Differential Revision: https://reviews.llvm.org/D158874