[RISCV] Teach RISCVGatherScatterLowering to handle more complex recurrence start values.

Previously we only recognized strided loads/store when the initial
value for the phi was a strided constant vector.

This patch extends the support to a strided_constant added to a
splatted value. The rewritten loop will add the splat value to the
first element of the strided constant vector to use as the scalar
start value. The stride is unaffected.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D115958