cmd/compile: number autotmps per-func, not per-package
Prior to this CL, autotmps were global to a package.
They also shared numbering with static variables.
Switch autotmp numbering to be per-function instead,
and do implicit numbering based on len(Func.Dcl).
This eliminates a dependency on a global variable
from the backend without adding to the Func struct.
While we're here, move statuniqgen closer to its
sole remaining user.
This actually improves compiler performance,
because the autotmp_* names can now be
reused across functions.