[BOLT] Tighter control of jump table detection

Summary:
We were too permissive by allowing more jump tables during the
preliminary scan of memory. This allowed for jump tables to be
falsely detected. And since we didn't have a way to backtrack
the jump table creation, we had to assert.

This diff refactors the code that analyzes jump table contents.
Preliminary and final passes share the same code. The only difference
should be the detection of instruction boundaries that are available
during the final pass.

This should affect strict relocation mode only.

(cherry picked from FBD16923335)