i965/fs: Emit better b2f of an expression on GEN4 and GEN5

i965/fs: Emit better b2f of an expression on GEN4 and GEN5

On platforms that do not natively generate 0u and ~0u for Boolean
results, b2f expressions that look like

    f = b2f(expr cmp 0)

will generate better code by pretending the expression is

    f = ir_triop_sel(0.0, 1.0, expr cmp 0)

This is because the last instruction of "expr" can generate the
condition code for the "cmp 0".  This avoids having to do the "-(b & 1)"
trick to generate 0u or ~0u for the Boolean result.  This means code like

    mov(16)         g16<1>F         1F
    mul.ge.f0(16)   null            g6<8,8,1>F      g14<8,8,1>F
    (+f0) sel(16)   m6<1>F          g16<8,8,1>F     0F

will be generated instead of

    mul(16)         g2<1>F          g12<8,8,1>F     g4<8,8,1>F
    cmp.ge.f0(16)   g2<1>D          g4<8,8,1>F      0F
    and(16)         g4<1>D          g2<8,8,1>D      1D
    and(16)         m6<1>D          -g4<8,8,1>D     0x3f800000UD

v2: When the comparison is either == 0.0 or != 0.0 use the knowledge
that the true (or false) case already results in zero would allow better
code generation by possibly avoiding a load-immediate instruction.

v3: Apply the optimization even when neither comparitor is zero.

Shader-db results:

GM45 (0x2A42):
total instructions in shared programs: 3551002 -> 3550829 (-0.00%)
instructions in affected programs:     33269 -> 33096 (-0.52%)
helped:                                121

Iron Lake (0x0046):
total instructions in shared programs: 4993327 -> 4993146 (-0.00%)
instructions in affected programs:     34199 -> 34018 (-0.53%)
helped:                                129

No change on other platforms.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Tapani Palli <tapani.palli@intel.com>