Change the register allocator so that it no longer tracks references

Change the register allocator so that it no longer tracks references
to the platform-specific reserved registers.  They are always in use
for their intended purpose, cannot appear in the virtual frame, and
can be freely used without allocation in the code generator.

Review URL: http://codereview.chromium.org/113837

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2061 ce2b1a6d-e550-0410-aec6-3dcde31c8c00