i965/fs: Track fixed GRF regs separate from allocated GRF file in scheduling.

i965/fs: Track fixed GRF regs separate from allocated GRF file in scheduling.

There's an assumption here that fixed GRFs will never intersect with
the allocated GRFs.  That's true today, though it might change some
day if we decide to register-allocate the regs containing push
constants once they're dead.

This fixes a regression in 0f7325b89038937bd428f7c89ed9859189a0ab0b in
Lightsmark from the texture instructions now containing g0 references
instead of having that be implied.  Performance is improved 15.2% +/-
3.6% (n=3).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=34968