Add TargetMachine::addEarlyAsPossiblePasses, and call it from opt.

Summary:
This is a hook to allow TargetMachine to install passes at the
EP_EarlyAsPossible PassManagerBuilder extension point.

Reviewers: chandlerc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D18614

llvm-svn: 267763