intel/vec4: Try to emit a VF source in try_immediate_source

intel/vec4: Try to emit a VF source in try_immediate_source

This commit is also a pre-requisite for the next commit.

No shader-db changes on any Gen8+ platform as these platforms do not use
the vec4 backend.

v2: Massive rebase on eeebeb211f1 ("intel/vec4: Try emitting non-scalar
immediates").  This change is a lot less helpful since that commit
landed (previously helped 1934 shaders on HSW) because, apparently, a
lot of the cases helped by that commit were things like vector loads of
{ 1.0, 1.0, 1.0 } that were also helped by this commit.

Haswell
total instructions in shared programs: 13480095 -> 13478598 (-0.01%)
instructions in affected programs: 229534 -> 228037 (-0.65%)
helped: 1006
HURT: 0
helped stats (abs) min: 1 max: 7 x̄: 1.49 x̃: 1
helped stats (rel) min: 0.04% max: 3.45% x̄: 1.11% x̃: 1.09%
95% mean confidence interval for instructions value: -1.54 -1.43
95% mean confidence interval for instructions %-change: -1.15% -1.07%
Instructions are helped.

total cycles in shared programs: 376385734 -> 376386916 (<.01%)
cycles in affected programs: 14101380 -> 14102562 (<.01%)
helped: 941
HURT: 56
helped stats (abs) min: 2 max: 322 x̄: 5.62 x̃: 2
helped stats (rel) min: <.01% max: 7.74% x̄: 0.51% x̃: 0.42%
HURT stats (abs)   min: 2 max: 618 x̄: 115.50 x̃: 32
HURT stats (rel)   min: 0.03% max: 4.62% x̄: 0.83% x̃: 0.44%
95% mean confidence interval for cycles value: -2.06 4.43
95% mean confidence interval for cycles %-change: -0.47% -0.39%
Inconclusive result (value mean confidence interval includes 0).

Ivy Bridge
total instructions in shared programs: 12048004 -> 12046589 (-0.01%)
instructions in affected programs: 217072 -> 215657 (-0.65%)
helped: 934
HURT: 0
helped stats (abs) min: 1 max: 7 x̄: 1.51 x̃: 1
helped stats (rel) min: 0.04% max: 3.45% x̄: 1.14% x̃: 1.11%
95% mean confidence interval for instructions value: -1.57 -1.46
95% mean confidence interval for instructions %-change: -1.18% -1.10%
Instructions are helped.

total cycles in shared programs: 180285854 -> 180287608 (<.01%)
cycles in affected programs: 14103824 -> 14105578 (0.01%)
helped: 871
HURT: 53
helped stats (abs) min: 2 max: 322 x̄: 5.51 x̃: 2
helped stats (rel) min: <.01% max: 7.67% x̄: 0.50% x̃: 0.42%
HURT stats (abs)   min: 2 max: 618 x̄: 123.66 x̃: 32
HURT stats (rel)   min: 0.03% max: 4.47% x̄: 0.92% x̃: 0.46%
95% mean confidence interval for cycles value: -1.60 5.39
95% mean confidence interval for cycles %-change: -0.46% -0.37%
Inconclusive result (value mean confidence interval includes 0).

Sandy Bridge
total instructions in shared programs: 10861227 -> 10860328 (<.01%)
instructions in affected programs: 92969 -> 92070 (-0.97%)
helped: 624
HURT: 0
helped stats (abs) min: 1 max: 7 x̄: 1.44 x̃: 1
helped stats (rel) min: 0.11% max: 3.45% x̄: 1.05% x̃: 0.95%
95% mean confidence interval for instructions value: -1.52 -1.36
95% mean confidence interval for instructions %-change: -1.09% -1.01%
Instructions are helped.

total cycles in shared programs: 153944316 -> 153942720 (<.01%)
cycles in affected programs: 1640956 -> 1639360 (-0.10%)
helped: 601
HURT: 15
helped stats (abs) min: 2 max: 120 x̄: 3.56 x̃: 2
helped stats (rel) min: 0.02% max: 6.33% x̄: 0.18% x̃: 0.08%
HURT stats (abs)   min: 2 max: 72 x̄: 36.13 x̃: 36
HURT stats (rel)   min: 0.05% max: 3.84% x̄: 1.95% x̃: 2.00%
95% mean confidence interval for cycles value: -3.44 -1.74
95% mean confidence interval for cycles %-change: -0.18% -0.09%
Cycles are helped.

Iron Lake and GM45 had similar results. (Iron Lake shown)
total instructions in shared programs: 8139924 -> 8139378 (<.01%)
instructions in affected programs: 69776 -> 69230 (-0.78%)
helped: 322
HURT: 0
helped stats (abs) min: 1 max: 8 x̄: 1.70 x̃: 1
helped stats (rel) min: 0.27% max: 3.23% x̄: 0.79% x̃: 0.54%
95% mean confidence interval for instructions value: -1.88 -1.51
95% mean confidence interval for instructions %-change: -0.85% -0.72%
Instructions are helped.

total cycles in shared programs: 188542864 -> 188541756 (<.01%)
cycles in affected programs: 3031532 -> 3030424 (-0.04%)
helped: 320
HURT: 0
helped stats (abs) min: 2 max: 20 x̄: 3.46 x̃: 2
helped stats (rel) min: <.01% max: 0.69% x̄: 0.06% x̃: 0.06%
95% mean confidence interval for cycles value: -3.85 -3.07
95% mean confidence interval for cycles %-change: -0.06% -0.05%
Cycles are helped.

Reviewed-by: Matt Turner <mattst88@gmail.com>