[RISCV] Optimize (add (mul r, c0), c1)

Optimize (add (mul x, c0), c1) ->
         (add (mul (add x, c1/c0+1), c0), c1%c0-c0),
if c1/c0+1 and c1%c0-c0 are simm12, while c1 is not.

Optimize (add (mul x, c0), c1) ->
         (add (mul (add x, c1/c0-1), c0), c1%c0+c0),
if c1/c0-1 and c1%c0+c0 are simm12, while c1 is not.

Reviewed By: craig.topper, asb

Differential Revision: https://reviews.llvm.org/D111141