TargetMachine: Indicate whether machine verifier passes.

This adds a callback to the LLVMTargetMachine that lets target indicate
that they do not pass the machine verifier checks in all cases yet.

This is intended to be a temporary measure while the targets are fixed
allowing us to enable the machine verifier by default with
EXPENSIVE_CHECKS enabled!

Differential Revision: https://reviews.llvm.org/D33696

llvm-svn: 304320