i965/fs: Properly patch special values during VGRF compaction.

i965/fs: Properly patch special values during VGRF compaction.

In addition to registers used by instructions, fs_visitor maintains
direct references to certain "special" values used for inputs/outputs.

When I added VGRF compaction, I overlooked these, believing that these
direct references weren't used once instructions were generated.  That
was wrong.  For example, pixel_x/y are used in virtual_grf_interferes(),
which is called by optimization passes and register allocation.

This patch treats all of them as used and patches them after compacting.
While it's not strictly necessary to patch all of them (as some aren't
used after emitting code), it seems safer to simply fix them all.

Fixes oglconform's textureswizzle/advanced.shader.targets, piglit's
glsl-fs-lots-of-tex, and glean's texCombine on pre-Gen6 hardware.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=56790
Reviewed-by: Eric Anholt <eric@anholt.net>