[PostRAMachineSink] preserve CFG

Summary: Mark CFG is preserved  since this pass do not make any change in CFG.

Reviewers: sebpop, mzolotukhin, mcrosier

Reviewed By: mzolotukhin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D44845

llvm-svn: 328727