[PowerPC] Fix machine verify pass error for PATCHPOINT pseudo instruction that bad machine code

Summary:
For SDAG, we pretend patchpoints aren't special at all until we emit the code for the pseudo.
Then the verifier runs and it seems like we have a use of an undefined register (the register will 
be reserved later, but the verifier doesn't know that).

So this patch call setUsesTOCBasePtr before emit the code for the pseudo, so verifier can know 
X2 is a reserved register.

Reviewed By: nemanjai

Differential Revision: https://reviews.llvm.org/D56148

llvm-svn: 350165