[RISCV] Merge addi into load/store as there is a ADD between them

This patch adds peephole optimizations for the following patterns:

(load (add base, (addi src, off1)), off2)
   -> (load (add base, src), off1+off2)
(store val, (add base, (addi src, off1)), off2)
   -> (store val, (add base, src), off1+off2)

Differential Revision: https://reviews.llvm.org/D124231