https://bugs.freedesktop.org/show_bug.cgi?id=99013
--- Comment #11 from Michel Dänzer michel@daenzer.net --- (In reply to Alexandre Demers from comment #9)
If we rely on features included after the 3.9 release, shouldn't mesa check for 4.0.0svn (at least, beginning from 4c8c13b3)?
The commit you bisected to does that, grep for HAVE_LLVM. However, there was a typo in the intrinsic name used with LLVM 3.9 and older.