rotate registers in regalloc on cfg:

[gretay-js]

Upstream compiler uses Proc.rotate_registers in coloring on Mach, but there is no corresponding optimization in IRC on CFG.
We are removing this unused setting from flambda-backend, but should consider implementing such an optimization on CFG in the future. Relevant comment removed from amd64 backend:

(* Pack registers starting at %rax so as to reduce the number of REX
   prefixes and thus improve code density *)
let rotate_registers = false