intel/nir: Combine store_derefs to improve code from SPIR-V

Due to lack of write mask in SPIR-V store, generators may produce
multiple stores to the same vector but using different array derefs.
Use the combining store pass to clean this up.  For example,

    layout(binding = 3) buffer block {
        vec4 v;
    };

    void main() {
        v.x = 11;
        v.y = 22;
    }

after going to SPIR-V and NIR, ends up with in two store_derefs to
v[0] and v[1]

    vec2 32 ssa_4 = deref_struct &ssa_3->field0 (ssbo vec4) /* &((block *)ssa_2)->field0 */
    vec2 32 ssa_6 = deref_array &(*ssa_4)[0] (ssbo float) /* &((block *)ssa_2)->field0[0] */
    intrinsic store_deref (ssa_6, ssa_7) (1, 0) /* wrmask=x */ /* access=0 */
    vec1 32 ssa_13 = load_const (0x00000001 /* 0.000000 */)
    vec2 32 ssa_14 = deref_array &(*ssa_4)[1] (ssbo float) /* &((block *)ssa_2)->field0[1] */
    intrinsic store_deref (ssa_14, ssa_15) (1, 0) /* wrmask=x */ /* access=0 */

producing two different sends instructions in skl.  The combining pass
transform the snippet above into

    vec2 32 ssa_4 = deref_struct &ssa_3->field0 (ssbo vec4) /* &((block *)ssa_2)->field0 */
    vec4 32 ssa_18 = vec4 ssa_7, ssa_15, ssa_16, ssa_17
    intrinsic store_deref (ssa_4, ssa_18) (3, 0) /* wrmask=xy */ /* access=0 */

producing a single sends instruction.

v2: Move this from spirv_to_nir into the general optimization pass for
    intel compiler.  (Jason)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/intel/compiler/brw_nir.c

@@ -570,6 +570,7 @@ brw_nir_optimize(nir_shader *nir, const struct brw_compiler *compiler,
       OPT(nir_copy_prop);
       OPT(nir_opt_dce);
       OPT(nir_opt_cse);
+      OPT(nir_opt_combine_stores, nir_var_all);
 
       /* Passing 0 to the peephole select pass causes it to convert
        * if-statements that contain only move instructions in the branches