[RISCV] Optimize (add (shl x, c0), (shl y, c1)) with SH*ADD

Optimize (add (shl x, c0), (shl y, c1)) ->
         (SLLI (SH*ADD x, y), c1), if c0-c1 == 1/2/3.

Reviewed By: craig.topper, luismarques

Differential Revision: https://reviews.llvm.org/D108916