[BOLT] Disassemble all functions before building CFGs

Summary:
This makes it possible to do adjustments to all functions based on
information gained during disassembly. E.g. if we detect an entry point
after the CFG for a function is constructed, we have to take a
conservative approach, and mark such function as non-simple. Now we have
this information before building the CFG. This could also be used to do
other processing/post-processing on disassembled functions that might
affect CFG construction of other functions (e.g. early detection of
functions that never return).

The drawback of this approach is that we lose cache locality and some
processing performance as a result. I've measured 5 second difference
on HHVM binary.

(cherry picked from FBD7258466)