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

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

This was reverted in r373454 due to breaking the expensive-checks bot.
This version addresses that by omitting the addSuccessorWithProb() call
when omitting the range check.

> 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: 373477