i965: Fold constants into the second arg of BRW_SEL as well.

This hits a common case with min/max operations.

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

@@ -2781,6 +2781,7 @@ fs_visitor::propagate_constants()
 	       }
 	       break;
 	    case BRW_OPCODE_CMP:
+	    case BRW_OPCODE_SEL:
 	       if (i == 1) {
 		  scan_inst->src[i] = inst->src[0];
 		  progress = true;