Check that the directory does not exist.
Otherwise, it could allows local users to obtain sensitive information or
overwrite arbitrary files via a symlink attack on temporary directories with
predictable names.

Reported as CVE-2014-2893 ( https://security-tracker.debian.org/tracker/CVE-2014-2893 )
Found by Jakub Wilk
Improve comments for r211040

Added comment to clarify why we r211040 choose to bail out of fast isel instead
of generating a more complicated relocation, and fix mislabelled register in the
comments of the asan test case.