tgsi_to_nir: Always use a NIR system value for front facing.

NIR always treats gl_FrontFacing as a boolean system value.  In GLSL IR,
we sometimes make it an input, and sometimes a system value, but either
way, glsl_to_nir turns it into a system value.  So, tgsi_to_nir should
not be generating NIR with input variables.

PIPE_CAP_TGSI_FS_FACE_IS_INTEGER_SYSVAL controls whether TGSI treats it
as a system value and expects reads to return an integer vector, or if
it treats it as an input and expects reads to return a float vector.

With this patch, in either case, we load a NIR system value, and produce
the floating point or integer vector as required by the capability bit.
21 jobs for !420 with ttn-frontface in 154 minutes and 8 seconds (queued for 6 minutes and 8 seconds)
latest detached
Status Job ID Name Coverage
  Containers Build
passed #162243
ubuntu

00:02:00

 
  Build+Test
passed #162257
make-gallium-drivers-other

00:14:03

passed #162256
make-gallium-drivers-radeonsi

00:08:40

passed #162255
make-gallium-drivers-swr

00:14:54

passed #162258
make-gallium-st-clover-llvm

00:13:57

passed #162259
make-gallium-st-clover-llvm-39

00:08:05

passed #162260
make-gallium-st-other

00:09:20

passed #162254
make-loader-classic-dri

00:12:52

passed #162253
make-vulkan

00:09:30

passed #162250
meson-gallium-clover-llvm

00:03:23

passed #162251
meson-gallium-clover-llvm39

00:02:57

passed #162249
meson-gallium-drivers-other

00:04:40

passed #162248
meson-gallium-radeonsi

00:02:37

passed #162252
meson-gallium-st-other

00:04:04

passed #162247
meson-gallium-swr

00:10:47

passed #162246
meson-glvnd

00:01:57

passed #162245
meson-loader-classic-dri

00:06:18

passed #162244
meson-vulkan

00:03:30

passed #162262
scons-llvm

00:05:11

passed #162261
scons-nollvm

00:06:14

passed #162263
scons-swr

00:08:58