Silence ubsan after r367926.

Fixes e.g.
http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-bootstrap-ubsan/builds/14273

We can't left shift here because left shifting of a negative number is UB.
The same doesn't apply to unsigned arithmetic, but switching to unsigned
doesn't appear to stop ubsan from complaining, so we need to mask out the
high bits.

llvm-svn: 367959