It appears to have caught a use-after-free introduced as by r159567
and/or friends which call 'addPass' from many more places. The bug in
'addPass' doesn't appear to be new, and was spotted by inspection when
ASan shown a bright light of a stacktrace at these functions.

Hopefully this will fix the ASan failure -- I have no test case other
than running an ASan-built clang over the test suite.

llvm-svn: 159614