intel/fs: Disable conditional discard optimization on Gen4 and Gen5

intel/fs: Disable conditional discard optimization on Gen4 and Gen5

The CMP instruction on Gen4 and Gen5 generates one bit (the LSB) of
valid data and 31 bits of junk.  Results of comparisons that are used as
Boolean values need to have a fixup applied to generate the proper 0/~0
values.

Calling fs_visitor::nir_emit_alu with need_dest=false prevents the fixup
code from being generated.  This results in a sequence like:

        cmp.l.f0.0(16)  g8<1>F          g14<8,8,1>F     0x0F  /* 0F */
        ...
        cmp.l.f0.0(16)  g4<1>F          g6<8,8,1>F      0x0F  /* 0F */
(+f0.1) or.z.f0.1(16) null<1>UD g4<8,8,1>UD     g8<8,8,1>UD

instead of

        cmp.l.f0.0(16)  g8<1>F          g14<8,8,1>F     0x0F  /* 0F */
        ...
        cmp.l.f0.0(16)  g4<1>F          g6<8,8,1>F      0x0F  /* 0F */
        or(16) g4<1>UD g4<8,8,1>UD     g8<8,8,1>UD
(+f0.1) and.z.f0.1(16) null<1>UD g4<8,8,1>UD     1UD

I examined a couple of the shaders hurt by this change, and ALL of them
would have been affected by this bug. :(

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1836
Fixes: 0ba9497e66a ("intel/fs: Improve discard_if code generation")
Iron Lake
total instructions in shared programs: 8122757 -> 8122957 (<.01%)
instructions in affected programs: 8307 -> 8507 (2.41%)
helped: 0
HURT: 100
HURT stats (abs)   min: 2 max: 2 x̄: 2.00 x̃: 2
HURT stats (rel)   min: 0.84% max: 6.67% x̄: 2.81% x̃: 2.76%
95% mean confidence interval for instructions value: 2.00 2.00
95% mean confidence interval for instructions %-change: 2.58% 3.03%
Instructions are HURT.

total cycles in shared programs: 188510100 -> 188510376 (<.01%)
cycles in affected programs: 76018 -> 76294 (0.36%)
helped: 0
HURT: 55
HURT stats (abs)   min: 2 max: 12 x̄: 5.02 x̃: 4
HURT stats (rel)   min: 0.07% max: 3.75% x̄: 0.86% x̃: 0.56%
95% mean confidence interval for cycles value: 4.33 5.71
95% mean confidence interval for cycles %-change: 0.60% 1.12%
Cycles are HURT.

GM45
total instructions in shared programs: 4994403 -> 4994503 (<.01%)
instructions in affected programs: 4212 -> 4312 (2.37%)
helped: 0
HURT: 50
HURT stats (abs)   min: 2 max: 2 x̄: 2.00 x̃: 2
HURT stats (rel)   min: 0.84% max: 6.25% x̄: 2.76% x̃: 2.72%
95% mean confidence interval for instructions value: 2.00 2.00
95% mean confidence interval for instructions %-change: 2.45% 3.07%
Instructions are HURT.

total cycles in shared programs: 128928750 -> 128928982 (<.01%)
cycles in affected programs: 67442 -> 67674 (0.34%)
helped: 0
HURT: 47
HURT stats (abs)   min: 2 max: 12 x̄: 4.94 x̃: 4
HURT stats (rel)   min: 0.09% max: 3.75% x̄: 0.75% x̃: 0.53%
95% mean confidence interval for cycles value: 4.19 5.68
95% mean confidence interval for cycles %-change: 0.50% 1.00%
Cycles are HURT.