Rework CFG simplification in bugpoint

Summary:
Depends on D22841

We now use a much simpler CFG simplification routine for bugpoint,
because SimplifyCFG is no longer a good match for what bugpoint wants
to do.

At the same time, to make sure we don't lose anything valuable it was doing,
SimplifyCFG is now run as a per-BB reduction pass.

With this and D22841 combined, bugpoint operates both much faster on
the large testcases i have, and reduces them to pretty much minimal
testcases (in one case, bugpoint used to leave about 6000 useless blocks, and
now it leaves 3 ...)

Reviewers: chandlerc, majnemer

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D22845

llvm-svn: 277063