Compute safety information in a much finer granularity.

Summary:
Instead of keeping a variable indicating whether there are early exits
in the loop.  We keep all the early exits. This improves LICM's ability to
move instructions out of the loop based on is-guaranteed-to-execute.

I am going to update compilation time as well soon.

Reviewers: hfinkel, sanjoy, efriedma, mkuper

Reviewed By: hfinkel

Subscribers: llvm-commits, mzolotukhin

Differential Revision: https://reviews.llvm.org/D32433

llvm-svn: 301196