i965/fs: Optimize OR with identical sources into a MOV.

Helps a lot of Steam games.

total instructions in shared programs: 1409360 -> 1409124 (-0.02%)
instructions in affected programs:     20842 -> 20606 (-1.13%)

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Paul Berry <stereotype441@gmail.com>

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

@@ -1872,6 +1872,14 @@ fs_visitor::opt_algebraic()
             break;
          }
          break;
+      case BRW_OPCODE_OR:
+         if (inst->src[0].equals(inst->src[1])) {
+            inst->opcode = BRW_OPCODE_MOV;
+            inst->src[1] = reg_undef;
+            progress = true;
+            break;
+         }
+         break;
       default:
 	 break;
       }