more sophisticated whole-bb reg spilling

_From [derek.br...@gmail.com](https://code.google.com/u/117968039472581148324/) on December 10, 2010 17:58:08_

PR 492073

this case covers implementing advanced features left out of PR 489221:
- if bb has high scores for all regs, try to split into 2: maybe 1st half
diff from 2nd half. Or, fall back to per-instr, can be less expensive
than restoring regs before each lea and app instr.
- If esi/edi/ebp are among the least-used, xchg w/ cx/dx/bx and swap in
each app instr. Have to ensure results in legal instrs (if app uses
sub-dword, or certain addressing modes).

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=163_