Avoid removing BBs referenced by JTs

Summary:
While removing unreachable blocks, we may decide to remove a
block that is listed as a target in a jump table entry. If we do that,
this label will be then undefined and LLVM assembler will crash.
Mitigate this for now by not removing such blocks, as we don't support
removing unnecessary jump tables yet.

Fixes facebookincubator/BOLT#20

(cherry picked from FBD8730269)