[PowerPC] Use add instead of addReg in ppc-early-ret pass

Summary:
The ppc-early-ret pass use the addReg() to add operand to the new
instruction, it can't reserve the flag of old operand. This has caused
machine verfications failed.
This patch use add() to instead of addReg().

Reviewed By: steven.zhang

Differential Revision: https://reviews.llvm.org/D77997