Number of registers on x86

[dewar@gnat.com]

The issue of how to do register allocation and scheduling in the presence
of renaming is a really interesting one, and has not been very much
addressed in the literature.

Conventional scheduling and register allocation can often make the
situation WORSE than what it would be in the hardware, since they
increase register pressure and cause spills, when in fact the conflicts
that are avoided are illusionary (since they would be eliminated by the
renaming).