st/nir: Free the GLSL IR after linking.

i965 does this, and st's tgsi path does this.  st/nir did not.

Cuts 138MB of memory from a DiRT Rally trace, which is about 44%
of the total GLSL IR memory.
15 jobs for !540 with st-no-glsl-ir in 15 minutes and 28 seconds (queued for 5 seconds)
latest detached
Status Job ID Name Coverage
  Containers Build
passed #199955
ubuntu

00:01:20

 
  Build+Test
passed #199956
autotools

00:13:23

passed #199958
meson-clang

00:08:57

passed #199964
meson-gallium-clover-llvm

00:06:52

passed #199965
meson-gallium-clover-llvm39

00:06:35

passed #199963
meson-gallium-drivers-other

00:06:45

passed #199962
meson-gallium-radeonsi

00:04:40

passed #199966
meson-gallium-st-other

00:05:46

passed #199957
meson-gallium-swr

00:11:43

passed #199961
meson-glvnd

00:05:05

passed #199960
meson-loader-classic-dri

00:08:11

passed #199959
meson-vulkan

00:03:25

passed #199968
scons-llvm

00:04:57

passed #199967
scons-nollvm

00:03:05

passed #199969
scons-swr

00:05:07