glsl: Eliminate assigments to out-of-bounds elements of vector

Several optimization paths, including constant folding, can lead to
indexing vector with an out of bounds index.

Out-of-bounds writes could be eliminated per spec:

Section 5.11 (Out-of-Bounds Accesses) of the GLSL 4.60 spec says:

"In the subsections described above for array, vector, matrix and
 structure accesses, any out-of-bounds access produced undefined
 behavior.... Out-of-bounds writes may be discarded or overwrite
 other variables of the active program."

Fixes piglit tests:
spec@glsl-1.20@execution@vector-out-of-bounds-access@fs-vec4-out-of-bounds-1
spec@glsl-1.20@execution@vector-out-of-bounds-access@fs-vec4-out-of-bounds-6

CC: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Marcin Ślusarz <marcin.slusarz@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6363>

.gitlab-ci/piglit/quick_shader.txt

@@ -369,8 +369,6 @@ spec/glsl-1.10/execution/built-in-functions/fs-pow-float-float: fail
 spec/glsl-1.10/execution/built-in-functions/vs-pow-float-float: fail
 spec/glsl-1.10/preprocessor/extension-defined-test: skip
 spec/glsl-1.10/preprocessor/extension-if-1: skip
-spec/glsl-1.20/execution/vector-out-of-bounds-access/fs-vec4-out-of-bounds-1: crash
-spec/glsl-1.20/execution/vector-out-of-bounds-access/fs-vec4-out-of-bounds-6: crash
 spec/glsl-1.30/execution/fs-texturegrad-miplevels: fail
 spec/glsl-1.30/execution/fs-texturelod-miplevels: fail
 spec/glsl-1.30/execution/fs-texturelod-miplevels-biased: fail
@@ -592,9 +590,9 @@ spec/nv_viewport_swizzle/viewport_swizzle: skip
 summary:
        name:  results
        ----  --------
-       pass:    15784
+       pass:    15786
        fail:      104
-      crash:      172
+      crash:      170
        skip:      315
     timeout:        0
        warn:        0