MFC r275633:

Pull in r223171 from upstream llvm trunk (by Michael Zolotukhin):

PR21302. Vectorize only bottom-tested loops.

rdar://problem/18886083

This fixes a bug in the llvm vectorizer, which could sometimes cause
vectorized loops to perform an additional iteration, leading to possible
buffer overruns. Symptoms of this, which are usually segfaults, were
first noticed when building gcc ports, here:

https://lists.freebsd.org/pipermail/freebsd-ports/2014-September/095466.html
https://lists.freebsd.org/pipermail/freebsd-toolchain/2014-September/001211.html

Since this fix is very important for ports, bump __FreeBSD_version to
make it easier for port maintainers to test whether the fix has been
applied.

Upstream PR: http://llvm.org/PR21302