[RISCV] Enable CGP to sink splat operands of Add/Sub/Mul/Shl/LShr/AShr

LICM may have pulled out a splat, but with .vx instructions we
can fold it into an operation.

This patch enables CGP to reverse the LICM transform and move the
splat back into the loop.

I've started with the commutable integer operations and shifts, but we can
extend this with more operations in future patches.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D109394