i965/fs: Fix register allocation for uniform pull constants in 16-wide.

i965/fs: Fix register allocation for uniform pull constants in 16-wide.

We were allowing a compressed instruction to write a register that
contained the last use of a uniform pull constant (either UBO load or push
constant spillover), so it would get half its values smashed.

Since we need to see the actual instruction to decide this, move the
pre-gen6 pixel_x/y logic here, which should improve the performance of
register allocation since virtual_grf_interferes() is called more than
once per instruction.

NOTE: This is a candidate for the stable branches.
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=61317
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>