intel/fs: Handle UNDEF in split_virtual_grfs

When the UNDEF instruction was added, we didn't do anything special in split_virtual_grfs. This mean that anything with an UNDEF wasn't getting split which causes problems for the compiler. Among other things, it makes RA harder because things are in bigger chunks. It also meant that dvec4s weren't getting split which means that they are larger than the maximum register size.

Shader-db results on Kaby Lake:

total instructions in shared programs: 14959202 -> 14960035 (<.01%)
instructions in affected programs: 96197 -> 97030 (0.87%)
helped: 140
HURT: 128
helped stats (abs) min: 1 max: 17 x̄: 1.62 x̃: 1
helped stats (rel) min: 0.09% max: 6.15% x̄: 0.65% x̃: 0.45%
HURT stats (abs)   min: 1 max: 825 x̄: 8.28 x̃: 1
HURT stats (rel)   min: 0.13% max: 139.83% x̄: 1.70% x̃: 0.50%
95% mean confidence interval for instructions value: -2.96 9.18
95% mean confidence interval for instructions %-change: -0.56% 1.51%
Inconclusive result (value mean confidence interval includes 0).

total loops in shared programs: 4372 -> 4372 (0.00%)
loops in affected programs: 0 -> 0
helped: 0
HURT: 0

total cycles in shared programs: 352646771 -> 352840997 (0.06%)
cycles in affected programs: 218600800 -> 218795026 (0.09%)
helped: 21167
HURT: 21411
helped stats (abs) min: 1 max: 2924 x̄: 36.89 x̃: 10
helped stats (rel) min: <.01% max: 41.90% x̄: 2.97% x̃: 0.98%
HURT stats (abs)   min: 1 max: 26027 x̄: 45.54 x̃: 10
HURT stats (rel)   min: <.01% max: 324.46% x̄: 3.88% x̃: 1.06%
95% mean confidence interval for cycles value: 2.87 6.26
95% mean confidence interval for cycles %-change: 0.40% 0.55%
Cycles are HURT.

total spills in shared programs: 8840 -> 8953 (1.28%)
spills in affected programs: 126 -> 239 (89.68%)
helped: 1
HURT: 2

total fills in shared programs: 21782 -> 21914 (0.61%)
fills in affected programs: 431 -> 563 (30.63%)
helped: 1
HURT: 3

LOST:   0
GAINED: 5

Shader-db results on Haswell:

total instructions in shared programs: 13320918 -> 13320769 (<.01%)
instructions in affected programs: 40998 -> 40849 (-0.36%)
helped: 146
HURT: 56
helped stats (abs) min: 1 max: 8 x̄: 2.73 x̃: 2
helped stats (rel) min: 0.16% max: 8.60% x̄: 2.52% x̃: 2.22%
HURT stats (abs)   min: 2 max: 23 x̄: 4.45 x̃: 4
HURT stats (rel)   min: 0.21% max: 10.26% x̄: 6.83% x̃: 10.26%
95% mean confidence interval for instructions value: -1.26 -0.21
95% mean confidence interval for instructions %-change: -0.62% 0.77%
Inconclusive result (%-change mean confidence interval includes 0).

total loops in shared programs: 4373 -> 4373 (0.00%)
loops in affected programs: 0 -> 0
helped: 0
HURT: 0

total cycles in shared programs: 374518258 -> 374384193 (-0.04%)
cycles in affected programs: 231101954 -> 230967889 (-0.06%)
helped: 21427
HURT: 19438
helped stats (abs) min: 1 max: 2035 x̄: 31.09 x̃: 8
helped stats (rel) min: <.01% max: 40.95% x̄: 2.42% x̃: 0.86%
HURT stats (abs)   min: 1 max: 20875 x̄: 27.38 x̃: 8
HURT stats (rel)   min: <.01% max: 59.09% x̄: 2.49% x̃: 0.80%
95% mean confidence interval for cycles value: -4.49 -2.07
95% mean confidence interval for cycles %-change: -0.14% -0.04%
Cycles are helped.

total spills in shared programs: 23406 -> 23411 (0.02%)
spills in affected programs: 3 -> 8 (166.67%)
helped: 0
HURT: 2

total fills in shared programs: 34845 -> 34850 (0.01%)
fills in affected programs: 3 -> 8 (166.67%)
helped: 0
HURT: 2

LOST:   0
GAINED: 0

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111566 Fixes: f4ef34f2 "intel/fs: Add an UNDEF instruction to avoid..."