i965/vec4: Emit smarter code for b2f of a comparison

i965/vec4: Emit smarter code for b2f of a comparison

Previously we would emit the comparison, emit an AND to mask off extra
bits from the comparison result, then convert the result to float.  Now,
do the comparison, then use a cleverly constructed SEL to pick either
0.0f or 1.0f.

No piglit regressions on Ivybridge.

total instructions in shared programs: 1642311 -> 1639449 (-0.17%)
instructions in affected programs:     136533 -> 133671 (-2.10%)
GAINED:                                0
LOST:                                  0

Programs that are affected appear to save between 1 and 5 instuctions
(just by skimming the output from shader-db report.py.

v2: s/b2i/b2f/ in commit subject (noticed by Chris Forbes).  Remove
extraneous fix_3src_operand (suggested by Matt).  The latter change
required swapping the order of the operands and using predicate_inverse.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>