[CodeGenPrepare] avoid crashing on non-canonical/degenerate code

The test is reduced from an example in the post-commit thread for:
rL354746
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20190304/632396.html

While we must avoid dying here, the real question should be:
Why is non-canonical and/or degenerate code making it to CGP when
using the new pass manager?

llvm-svn: 355345