Update the register allocator document.
Update the register allocator document.

- We have a coalescing pass which puts the code in CSSA form.
- We do not have an EBB argument fixup pass. This isn't needed with
  CSSA.

stoklund/cretonne,stoklund/cretonne,stoklund/cretonne,sunfishcode/cretonne,sunfishcode/cretonne,sunfishcode/cretonne