RegAlloc: Account for a variable entry block frequency

Until r197284, the entry frequency was constant -- i.e., set to 2^14.
Although current ToT still has a constant entry frequency, since r197284
that has been an implementation detail (which is soon going to change).

  - r204690 made the wrong assumption for the CSRCost metric.  Adjust
    callee-saved register cost based on entry frequency.

  - r185393 made the wrong assumption (although it was valid at the
    time).  Update SpillPlacement.cpp::Threshold to be relative to the
    entry frequency.

Since ToT still has 2^14 entry frequency, this should have no observable
functionality change.

<rdar://problem/14292693>