i965: Don't consider null dst instructions as matching non-null dst.

When performing common subexpression elimination on instructions with
non-null destinations we emit a MOV to copy the result to a new
register that must have no other uses. In the case of:

   cmp.g.f0.0(8) null:D, vgrf43:F, 0.500000f
   ...
   cmp.g.f0.0(8) vgrf113:D, vgrf43:F, 0.500000f

we put the first instruction in the AEB and decided that we could reuse
its result when we found the second. Unfortunately, that meant that we'd
emit a MOV from the first's destination, which is null.

Don't do anything if the entry's destination is null and the
instruction's destination is non-null.

Tested-by: Tapani Pälli <tapani.palli@intel.com>

src/mesa/drivers/dri/i965/brw_fs_cse.cpp

@@ -194,7 +194,8 @@ fs_visitor::opt_cse_local(bblock_t *block)
 
          foreach_in_list_use_after(aeb_entry, entry, &aeb) {
             /* Match current instruction's expression against those in AEB. */
-            if (instructions_match(inst, entry->generator)) {
+            if (!(entry->generator->dst.is_null() && !inst->dst.is_null()) &&
+                instructions_match(inst, entry->generator)) {
                found = true;
                progress = true;
                break;

src/mesa/drivers/dri/i965/brw_vec4_cse.cpp

@@ -157,7 +157,8 @@ vec4_visitor::opt_cse_local(bblock_t *block)
 
          foreach_in_list_use_after(aeb_entry, entry, &aeb) {
             /* Match current instruction's expression against those in AEB. */
-            if (instructions_match(inst, entry->generator)) {
+            if (!(entry->generator->dst.is_null() && !inst->dst.is_null()) &&
+                instructions_match(inst, entry->generator)) {
                found = true;
                progress = true;
                break;