[RISCV] Handle non-recursive muls of strides in gather/scatter lowering

The gather scatter lowering pass can fold multiplies of a step vector
into the stride for the recursive case, so this extends it for the
non-recursive case.
The logic can probably be shared between the two at some point to extend
it to shls and ors.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D146983