i965/fs: Handle CMP.nz ... 0 and AND.nz ... 1 similarly in cmod propagation

i965/fs: Handle CMP.nz ... 0 and AND.nz ... 1 similarly in cmod propagation

Espically on platforms that do not natively generate 0u and ~0u for
Boolean results, we generate a lot of sequences where a CMP is
followed by an AND with 1.  emit_bool_to_cond_code does this, for
example.  On ILK, this results in a sequence like:

    add(8)          g3<1>F          g8<8,8,1>F      -g4<0,1,0>F
    cmp.l.f0(8)     g3<1>D          g3<8,8,1>F      0F
    and.nz.f0(8)    null            g3<8,8,1>D      1D
    (+f0) iff(8)    Jump: 6

The AND.nz is obviously redundant.  By propagating the cmod, we can
instead generate

    add.l.f0(8)     null            g8<8,8,1>F      -g4<0,1,0>F
    (+f0) iff(8)    Jump: 6

Existing code already handles the propagation from the CMP to the ADD.

Shader-db results:

GM45 (0x2A42):
total instructions in shared programs: 3550829 -> 3550788 (-0.00%)
instructions in affected programs:     10028 -> 9987 (-0.41%)
helped:                                24

Iron Lake (0x0046):
total instructions in shared programs: 4993146 -> 4993105 (-0.00%)
instructions in affected programs:     9675 -> 9634 (-0.42%)
helped:                                24

Ivy Bridge (0x0166):
total instructions in shared programs: 6291870 -> 6291794 (-0.00%)
instructions in affected programs:     17914 -> 17838 (-0.42%)
helped:                                48

Haswell (0x0426):
total instructions in shared programs: 5779256 -> 5779180 (-0.00%)
instructions in affected programs:     16694 -> 16618 (-0.46%)
helped:                                48

Broadwell (0x162E):
total instructions in shared programs: 6823088 -> 6823014 (-0.00%)
instructions in affected programs:     15824 -> 15750 (-0.47%)
helped:                                46

No chage on Sandy Bridge or on any platform when NIR is used.

v2: Add unit tests suggested by Matt.  Remove spurious writes_flag()
check on scan_inst when scan_inst is known to be BRW_OPCODE_CMP (also
suggested by Matt).

v3: Fix some comments and remove some explicit int() casts in fs_reg
constructors in the unit tests.  Both suggested by Matt.

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