[RISCV] Scalarize constant stores of fixed vectors if small enough

For stores of small fixed-length vector constants, we can store them
with a sequence of lui/addi/sh/sw to avoid the cost of building the
vector and the vsetivli toggle, provided the constant materialization
cost isn't too high.

This subsumes the optimisation for stores of zeroes in
4dc9a2c5

(This is a reapply of 0ca13f9d)

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D151221