CodeGen: Add a stand-alone hazard recognizer pass

Summary:
This new pass allows targets to use the hazard recognizer without having
to also run one of the schedulers.  This is useful when compiling with
optimizations disabled for targets that still need noop hazards
to be handled correctly.

Reviewers: hfinkel, atrick

Subscribers: arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D18594

llvm-svn: 267156