Revert r373431 "Switch lowering: omit range check for bit tests when default...

Revert r373431 "Switch lowering: omit range check for bit tests when default is unreachable (PR43129)"

This broke http://lab.llvm.org:8011/builders/llvm-clang-x86_64-expensive-checks-win/builds/19967

> Switch lowering: omit range check for bit tests when default is unreachable (PR43129)
>
> This is modeled after the same functionality for jump tables, which was
> added in r357067.
>
> Differential revision: https://reviews.llvm.org/D68131

llvm-svn: 373454