[PowerPC] Enable machine verification for 3 passes

Summary:
For PowerPC, there are 3 passes has disabled the machine verification.
```
PPCTargetMachine.cpp:    addPass(&LiveVariablesID, false);
PPCTargetMachine.cpp:    addPass(createPPCEarlyReturnPass(), false);
PPCTargetMachine.cpp:  addPass(createPPCBranchSelectionPass(), false);
```
This patch is to enable machine verification for above three passes.

Reviewed By: steven.zhang

Differential Revision: https://reviews.llvm.org/D79840