[ISEL][BitTestBlock] pre-commit test for D109103

Upload a test that shows ISEL taking a SwitchInst that has an
unreachable BB for a default target being lowered to an unconditional
jump off the end of a function.

Link: https://bugs.llvm.org/show_bug.cgi?id=50080
Link: https://github.com/ClangBuiltLinux/linux/issues/679
Link: https://github.com/ClangBuiltLinux/linux/issues/1440

Reviewed By: craig.topper, hans

Differential Revision: https://reviews.llvm.org/D109106