i965/vec4: Always use NIR

i965/vec4: Always use NIR

GLSL IR vs. NIR shader-db results for vec4 programs on i965:

   total instructions in shared programs: 1499328 -> 1388354 (-7.40%)
   instructions in affected programs:     1245199 -> 1134225 (-8.91%)
   helped:                                7469
   HURT:                                  2440

GLSL IR vs. NIR shader-db results for vec4 programs on G4x:

   total instructions in shared programs: 1436799 -> 1325825 (-7.72%)
   instructions in affected programs:     1205599 -> 1094625 (-9.20%)
   helped:                                7469
   HURT:                                  2440

GLSL IR vs. NIR shader-db results for vec4 programs on Iron Lake:

   total instructions in shared programs: 1436654 -> 1325682 (-7.72%)
   instructions in affected programs:     1205503 -> 1094531 (-9.21%)
   helped:                                7468
   HURT:                                  2440

GLSL IR vs. NIR shader-db results for vec4 programs on Sandy Bridge:

   total instructions in shared programs: 2016249 -> 1787033 (-11.37%)
   instructions in affected programs:     1850547 -> 1621331 (-12.39%)
   helped:                                14856
   HURT:                                  1481

GLSL IR vs. NIR shader-db results for vec4 programs on Ivy Bridge:

   total instructions in shared programs: 1848027 -> 1648216 (-10.81%)
   instructions in affected programs:     1660279 -> 1460468 (-12.03%)
   helped:                                14668
   HURT:                                  1369

GLSL IR vs. NIR shader-db results for vec4 programs on Bay Trail:

   total instructions in shared programs: 1848027 -> 1648216 (-10.81%)
   instructions in affected programs:     1660279 -> 1460468 (-12.03%)
   helped:                                14668
   HURT:                                  1369

GLSL IR vs. NIR shader-db results for vec4 programs on Haswell:

   total instructions in shared programs: 1848027 -> 1648216 (-10.81%)
   instructions in affected programs:     1660279 -> 1460468 (-12.03%)
   helped:                                14668
   HURT:                                  1369

I also ran our full suite of benchmarks on a Haswell and had the following
statistically significant (according to ministat) changes:

   Test                        master-glsl     master-nir     diff
   bench_OglGeomPoint          461.556         463.006        1.450
   bench_OglTerrainFlyInst     184.484         187.574        3.090
   bench_OglTerrainPanInst     132.412         136.307        3.895
   bench_OglTexFilterAniso     19.653          19.645         -0.008
   bench_OglTexFilterTri       58.333          58.009         -0.324
   bench_OglVSInstancing       65.049          65.327         0.278
   bench_trexoff               69.474          69.694         0.220
   bench_valley                40.708          41.125         0.417

v2 (Jason Ekstrand):
 - Remove more uses of NirOptions as a switch
 - New shader-db numbers
 - Added benchmark numbers

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