[RISCV] Use scalar stores for splats of zero to memory up to XLen

The direct motivation here is to undo an unprofitable vectorization performed by SLP, but the transform seems generally useful as well. If we are storing a zero to memory, we can use a single scalar store (from X0) for all power of two sizes up to XLen.

Differential Revision: https://reviews.llvm.org/D150717