Bail out of a SimplifyCFG switch table opt at undef values.

Summary:
A true or false result is expected from a comparison, but it seems the possibility of undef was overlooked, which could lead to a failed assert. This is fixed by this patch by bailing out if we encounter undef.

The bug is old and the assert has been there since the end of 2014, so it seems this is unusual enough to forego optimization.

Patch by: JesperAntonsson

Reviewers: spatel, eeckstein, hans

Reviewed By: hans

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D40639

llvm-svn: 319537