[RISCV] Teach RISCVMergeBaseOffset about cases where we use SHXADD to add some immediates.

For an addition with simm14 and simm15 immediates with 2 or 3 trailing bits,
we can use a shXadd instruction and an addi to do the addition.

This patch teaches RISCVMergeBaseOffset to see through this pattern.
I don't think the sh1add case occurs because we use two addis for that,
but I implemented it for completeness.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D127376