[x86] Fix a bug in r216319 where I was missing a 'break'.

This actually was caught by existing tests but those tests were disabled
with an XFAIL because of PR20736. While working on fixing that,
I noticed the test failure, and tracked it down to this.

We even have a really nice Clang warning that would have caught this but
it isn't enabled in LLVM! =[ I may look at enabling it.