nir: get rid of OOB dereferences in nir_lower_io_arrays_to_elements

This patch fixes mesa compiler crash in i965 on shaders like the following one:
```
   in VS_OUTPUT {
      mat4 data;
   } vs_output;
   out vec4 fs_output;

   vec4 convert(in float val) {
       return vec4(val);
   }

   void main()
   {
       fs_output = vec4(0.0);
       for (int a = -1; a < 5; a++) {
           for (int b = -1; b < 5; b++) {
               fs_output += convert(vs_output.data[b][a]);
           }
       }
   }
```

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 reads return undefined values, which
   include values from other variables of the active program or zero.
   Out-of-bounds writes may be discarded or overwrite
   other variables of the active program.

GL_KHR_robustness and GL_ARB_robustness encourage us to return zero
for reads.

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Signed-off-by: Andrii Simiklit <andrii.simiklit@globallogic.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6560>

src/compiler/nir/nir_lower_io_arrays_to_elements.c

@@ -115,6 +115,18 @@ lower_array(nir_builder *b, nir_intrinsic_instr *intr, nir_variable *var,
 {
    b->cursor = nir_before_instr(&intr->instr);
 
+   if (nir_deref_instr_is_known_out_of_bounds(nir_src_as_deref(intr->src[0]))) {
+      /* See Section 5.11 (Out-of-Bounds Accesses) of the GLSL 4.60 */
+      if (intr->intrinsic != nir_intrinsic_store_deref) {
+         nir_ssa_def *zero = nir_imm_zero(b, intr->dest.ssa.num_components,
+                                          intr->dest.ssa.bit_size);
+         nir_ssa_def_rewrite_uses(&intr->dest.ssa,
+                                  nir_src_for_ssa(zero));
+      }
+      nir_instr_remove(&intr->instr);
+      return;
+   }
+
    nir_variable **elements =
       get_array_elements(varyings, var, b->shader->info.stage);