Add initial framework for PGO in the JIT, and use it for trace-region selection

Add initial framework for PGO in the JIT, and use it for trace-region selection

This diff adds an initial framework for profile-guided optimizations
in the JIT.  Two new translation modes are added, effectively creating
a multi-gear JIT:

  1. Profile: this collects profiling data, which includes execution
              counters and type information.
  2. Optimize: this uses data collected by Profiling translations to
               produce optimized translations.

Right now, the Optimize gear is solely used to produce larger
compilation regions (traces), which increase the scope of
optimizations exposed to the JIT (compared to tracelets).  This is
still work in progress, and it's disabled by default.

This diff also fixes a number of bugs exposed by trace regions, gets
tools/reduce working again, and makes a number of improvements to
tc-print related to the new translation modes.