nir/lower_vec_to_movs: Don't emit unneeded movs

It's possible that, if a vecN operation is involved in a phi node, that we
could end up moving from a register to itself.  If swizzling is involved,
we need to emit the move but.  However, if there is no swizzling, then the
mov is a no-op and we might as well not bother emitting it.

Shader-db results on Haswell:

   total instructions in shared programs: 6262536 -> 6259558 (-0.05%)
   instructions in affected programs:     184780 -> 181802 (-1.61%)
   helped:                                838
   HURT:                                  0

Reviewed-by: Eduardo Lima Mitev <elima@igalia.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/glsl/nir/nir_lower_vec_to_movs.c

@@ -83,7 +83,25 @@ insert_mov(nir_alu_instr *vec, unsigned start_idx, nir_shader *shader)
       }
    }
 
-   nir_instr_insert_before(&vec->instr, &mov->instr);
+   /* In some situations (if the vecN is involved in a phi-web), we can end
+    * up with a mov from a register to itself.  Some of those channels may end
+    * up doing nothing and there's no reason to have them as part of the mov.
+    */
+   if (src_matches_dest_reg(&mov->dest.dest, &mov->src[0].src) &&
+       !mov->src[0].abs && !mov->src[0].negate) {
+      for (unsigned i = 0; i < 4; i++) {
+         if (mov->src[0].swizzle[i] == i) {
+            mov->dest.write_mask &= ~(1 << i);
+         }
+      }
+   }
+
+   /* Only emit the instruction if it actually does something */
+   if (mov->dest.write_mask) {
+      nir_instr_insert_before(&vec->instr, &mov->instr);
+   } else {
+      ralloc_free(mov);
+   }
 
    return mov->dest.write_mask;
 }