nir: Allow zero-length array copies

For drivers that have a standalone compiler like lima, when we compile
just a vertex shader we have to not use the linker, lest everything get
removed since it thinks nothing is in use. In legacy GL there's
gl_TexCoord, which is sized implicitly based on the maximum index used
to store it. Normally if it's not stored to it's removed by the linker,
but since we don't use the linker and lima uses lower_io_to_temporaries,
we get a zero-length copy of gl_TexCoord. There was an assertion against
this in NIR, but the code handles it perfectly fine. Just remove it.
22 jobs for !1836 with nir-zero-len-copies in 14 minutes and 25 seconds (queued for 4 seconds)
latest detached
Status Job ID Name Coverage
  Containers Build
passed #557830
debian

00:00:19

 
  Build+Test
passed #557839
meson-arm64

00:01:32

passed #557838
meson-armhf

00:01:26

passed #557832
meson-clang

00:04:07

passed #557836
meson-clover

00:05:05

passed #557840
meson-i386

00:02:25

passed #557835
meson-main

00:09:18

passed #557831
meson-swr-glvnd

00:06:15

passed #557837
meson-vulkan

00:02:01

passed #557842
scons-llvm

00:03:54

passed #557841
scons-nollvm

00:05:41

passed #557833
scons-swr

00:05:08

passed #557834
scons-win64

00:01:54

 
  Test
passed #557843
test-llvmpipe-gles2 1/4

00:03:27

passed #557844
test-llvmpipe-gles2 2/4

00:03:41

passed #557845
test-llvmpipe-gles2 3/4

00:03:25

passed #557846
test-llvmpipe-gles2 4/4

00:03:36

passed #557847
test-softpipe-gles2 1/4

00:01:25

passed #557848
test-softpipe-gles2 2/4

00:01:32

passed #557849
test-softpipe-gles2 3/4

00:01:24

passed #557850
test-softpipe-gles2 4/4

00:01:26

passed #557851
test-softpipe-gles3-limited

00:03:50