BOLT: Remove restrictions on unreachable code elimination

Summary:
Allow UCE when blocks have EH info.  Since UCE may remove blocks
that are referenced from debugging info data structures, we don't
actually delete them.  We just mark them with an "invalid" index
and store them in a different vector to be cleaned up later once
the BinaryFunction is destroyed.  The debugging code just skips
any BBs that have an invalid index.

Eliminating blocks may also expose useless jmp instructions, i.e.
a jmp around a dead block could just be a fallthrough.  I've added
a new routine to cleanup these jmps.  Although, @maks is working on
changing fixBranches() so that it can be used instead.

(cherry picked from FBD3793259)