[RISCV] Fold store of vmv.x.s to a vse with VL=1.

This can avoid a loss of decoupling with the scalar unit on cores
with decoupled scalar and vector units.

We should support FP too, but those use extract_element and not a
custom ISD node so it is a little different. I also left a FIXME
in the test for i64 extract and store on RV32.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D109482