[PowerPC] Remove the redundant implicit operands in ppc-early-ret pass

Summary:
In the `ppc-early-ret` pass, we have use `BuildMI` and `copyImplicitOps` when the branch instructions can do the early return. But the two functions will add implicit operands twice, this is not correct.

This patch is to remove the redundant implicit operands in `ppc-early-ret pass`.

Reviewed By: jsji

Differential Revision: https://reviews.llvm.org/D76042