[NFCI] Change RewriteStatepointsForGC to a ModulePass.

Summary:
A later change that has RewriteStatepointsForGC change function
attributes throughout the module depends on this.

Reviewers: reames, pgavlin

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10104

llvm-svn: 238882