[RISCV] Scalarize small fixed vector copies < XLEN

For small fixed-length vector copies like
vsetivli   zero, 2, e16, m1, ta, ma
vle16.v    v8, (a0)
vse16.v    v8, (a1)

We can scalarize them if the total vector size < XLEN:
lw a0, 0(a0)
sw a0, 0(a1)

This patch adds a DAG combine to do so, reusing much of the existing
logic in https://reviews.llvm.org/D150717

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D151103