PMB: Run the whole-program-devirt pass during LTO at --lto-O0.

The whole-program-devirt pass needs to run at -O0 because only it
knows about the llvm.type.checked.load intrinsic: it needs to both
lower the intrinsic itself and handle it in the summary.

Differential Revision: https://reviews.llvm.org/D33571

llvm-svn: 304019