[RISCV] Implement storeOfVectorConstantIsCheap hook to prevent store merging at VL=2

In general, VL=2 vectors are very questionable profitability wise. For constants specifically, our inability to materialize many vector constants cheaply biases us strongly towards unprofitability at VL=2.

This hook is very close to the x86 implementation. The difference is that X86 whitelists stores of zeros, and we're better off letting that stay scalar at VL=2.

Differential Revision: https://reviews.llvm.org/D150798