glsl: change variable declared assertion into conditional

The slang_variable::declared field originated as a debug field but
can be promoted for use during sematic error checking.

Fixes fd.o bug 27921.

NOTE: this is a candidate for back-porting to the 7.8 stable branch.

src/mesa/shader/slang/slang_codegen.c

    slang_atom name = oper->var ? oper->var->a_name : oper->a_id;
    slang_variable *var = _slang_variable_locate(oper->locals, name, GL_TRUE);
    slang_ir_node *n;
-   if (!var) {
+   if (!var || !var->declared) {
       slang_info_log_error(A->log, "undefined variable '%s'", (char *) name);
       return NULL;
    }
-   assert(var->declared);
    n = new_var(A, var);
    return n;
 }

src/mesa/shader/slang/slang_compile_variable.h

    GLuint size;                     /**< Variable's size in bytes */
    GLboolean is_global;
    GLboolean isTemp;                /**< a named temporary (__resultTmp) */
-   GLboolean declared;              /**< for debug */
+   GLboolean declared;              /**< has the var been declared? */
    struct slang_ir_storage_ *store; /**< Storage for this var */
 } slang_variable;