[PowerPC] fold addi's imm operand to its imm form consumer's displacement

This patch adds a function to do following transformation:

%0:g8rc_and_g8rc_nox0 = ADDI8 %5:g8rc_and_g8rc_nox0, 144
STD killed %7:g8rc, 16, %0:g8rc_and_g8rc_nox0 :: (store 8 into %ir.8)

------>

STD killed %7:g8rc, 160, %5:g8rc_and_g8rc_nox0 :: (store 8 into %ir.8)

Reviewed By: steven.zhang

Differential Revision: https://reviews.llvm.org/D81723