i965/fs: Fix register comparisons in saturate propagation.

i965/fs: Fix register comparisons in saturate propagation.

opt_saturate_propagation_local compares scan_inst->dst.reg/reg_offset
with inst->src[0].reg/reg_offset, and ensures that scan_inst->dst.file
is GRF.  But nothing ensured that inst->src[0].file was GRF.

In the following program, this resulted in u1:F matching vgrf1:UW,
and a saturate being incorrectly propagated from instruction 8 to
instruction 1.

{  1}    0: add vgrf0:UW, hw_reg1+8:UW, hw_reg0:V
{  1}    1: add vgrf1:UW, hw_reg1+10:UW, hw_reg0:V
{  1}    2: linterp vgrf6:F, hw_reg2:F, hw_reg3:F, hw_reg0:F
{  2}    3: linterp vgrf27:F, hw_reg2:F, hw_reg3:F, hw_reg0+16:F
{  4}    4: mov vgrf10+0.0:F, vgrf6:F
{  3}    5: mov vgrf10+1.0:F, vgrf27:F
{  6}    6: tex vgrf8+0.0:F, vgrf10+0.0:F
{  5}    7: mov vgrf32:F, u1:F
{  5}    8: mov.sat vgrf12:F, u1:F

From shader-db:
   total instructions in shared programs: 1841932 -> 1841957 (0.00%)
   instructions in affected programs:     5823 -> 5848 (0.43%)
I inspected two of the 25 hurt shaders, and concluded that they were
both hitting this bug, and not legitimately optimized.

This fixes bugs in Left 4 Dead 2 and Team Fortress 2, possibly among
others.  The optimization pass didn't exist in 10.0, so this is only
a candidate for 10.1.

Cc: "10.1" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>