Currently the liveness analysis pass would extend a live interval up
to the top of the program when no unconditional and complete
definition of the variable is found that dominates all of its uses.

This can lead to a serious performance problem in shaders containing
many partial writes, like scalar arithmetic, FP64 and soon FP16
operations.  The number of oversize live intervals in such workloads
can cause the compilation time of the shader to explode because of the
worse than quadratic behavior of the register allocator and scheduler
when running out of registers, and it can also cause the running time
of the shader to explode due to the amount of spilling it leads to,
which is orders of magnitude slower than GRF memory.

This patch fixes it by computing the intersection of our current live
intervals with the subset of the program that can possibly be reached
from any definition of the variable.  Extending the storage allocation
of the variable beyond that is pretty useless because its value is
guaranteed to be undefined at a point that cannot be reached from any
definition.

According to Jason, this improves performance of the subgroup Vulkan
CTS tests significantly (e.g. the runtime of the dvec4 broadcast test
improves by nearly 50x).

No significant change in the running time of shader-db (with 5%
statistical significance).

shader-db results on IVB:

  total cycles in shared programs: 61108780 -> 60932856 (-0.29%)
  cycles in affected programs: 16335482 -> 16159558 (-1.08%)
  helped: 5121
  HURT: 4347

  total spills in shared programs: 1309 -> 1288 (-1.60%)
  spills in affected programs: 249 -> 228 (-8.43%)
  helped: 3
  HURT: 0

  total fills in shared programs: 1652 -> 1597 (-3.33%)
  fills in affected programs: 262 -> 207 (-20.99%)
  helped: 4
  HURT: 0

  LOST:   2
  GAINED: 209

shader-db results on BDW:

  total cycles in shared programs: 67617262 -> 67361220 (-0.38%)
  cycles in affected programs: 23397142 -> 23141100 (-1.09%)
  helped: 8045
  HURT: 6488

  total spills in shared programs: 1456 -> 1252 (-14.01%)
  spills in affected programs: 465 -> 261 (-43.87%)
  helped: 3
  HURT: 0

  total fills in shared programs: 1720 -> 1465 (-14.83%)
  fills in affected programs: 471 -> 216 (-54.14%)
  helped: 4
  HURT: 0

  LOST:   2
  GAINED: 162

shader-db results on SKL:

  total cycles in shared programs: 65436248 -> 65245186 (-0.29%)
  cycles in affected programs: 22560936 -> 22369874 (-0.85%)
  helped: 8457
  HURT: 6247

  total spills in shared programs: 437 -> 437 (0.00%)
  spills in affected programs: 0 -> 0
  helped: 0
  HURT: 0

  total fills in shared programs: 870 -> 854 (-1.84%)
  fills in affected programs: 16 -> 0
  helped: 1
  HURT: 0

  LOST:   0
  GAINED: 107

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>