i965: Revert part of f5cc3fdcf1.

Fixes non-termination in various Piglit tests.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>

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

@@ -82,7 +82,7 @@ is_expression(const vec4_instruction *const inst)
    case SHADER_OPCODE_COS:
       return inst->mlen == 0;
    default:
-      return !inst->has_side_effects();
+      return false;
    }
 }