i965/vec4: Generate better code for ir_triop_csel.

i965/vec4: Generate better code for ir_triop_csel.

Previously, we generated an extra CMP instruction:

   cmp.ge.f0(8)    g6<1>D          g1<0,4,1>F      0F
   cmp.nz.f0(8)    null            g6<4,4,1>D      0D
   (+f0) sel(8)    g5<1>F          g1.4<0,4,1>F    g2<0,4,1>F

The first operand is always a boolean, and we want to predicate the SEL
on that.  Rather than producing a boolean value and comparing it against
zero, we can just produce a condition code in the flag register.

Now we generate:

   cmp.ge.f0(8)    null            g1<0,4,1>F      0F
   (+f0) sel(8)    g5<1>F          g1.4<0,4,1>F    g2<0,4,1>F

No difference in shader-db.

v2: Remember to delete the old code (thanks Matt).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>