py/compile: Optimise emitter label indices to save a word of heap.

Previous to this patch, a label with value "0" was used to indicate an
invalid label, but that meant a wasted word (at slot 0) in the array of
label offsets.  This patch adjusts the label indices so the first one
starts at 0, and the maximum value indicates an invalid label.

py/compile.c

@@ -40,6 +40,8 @@
 
 // TODO need to mangle __attr names
 
+#define INVALID_LABEL (0xffff)
+
 typedef enum {
 // define rules with a compile function
 #define DEF_RULE(rule, comp, kind, ...) PN_##rule,
@@ -954,7 +956,7 @@ STATIC void compile_del_stmt(compiler_t *comp, mp_parse_node_struct_t *pns) {
 }
 
 STATIC void compile_break_stmt(compiler_t *comp, mp_parse_node_struct_t *pns) {
-    if (comp->break_label == 0) {
+    if (comp->break_label == INVALID_LABEL) {
         compile_syntax_error(comp, (mp_parse_node_t)pns, "'break' outside loop");
     }
     assert(comp->cur_except_level >= comp->break_continue_except_level);
@@ -962,7 +964,7 @@ STATIC void compile_break_stmt(compiler_t *comp, mp_parse_node_struct_t *pns) {
 }
 
 STATIC void compile_continue_stmt(compiler_t *comp, mp_parse_node_struct_t *pns) {
-    if (comp->continue_label == 0) {
+    if (comp->continue_label == INVALID_LABEL) {
         compile_syntax_error(comp, (mp_parse_node_t)pns, "'continue' outside loop");
     }
     assert(comp->cur_except_level >= comp->break_continue_except_level);
@@ -2960,7 +2962,7 @@ STATIC void check_for_doc_string(compiler_t *comp, mp_parse_node_t pn) {
 STATIC void compile_scope(compiler_t *comp, scope_t *scope, pass_kind_t pass) {
     comp->pass = pass;
     comp->scope_cur = scope;
-    comp->next_label = 1;
+    comp->next_label = 0;
     EMIT_ARG(start_pass, pass, scope);
 
     if (comp->pass == MP_PASS_SCOPE) {
@@ -3135,7 +3137,7 @@ STATIC void compile_scope(compiler_t *comp, scope_t *scope, pass_kind_t pass) {
 STATIC void compile_scope_inline_asm(compiler_t *comp, scope_t *scope, pass_kind_t pass) {
     comp->pass = pass;
     comp->scope_cur = scope;
-    comp->next_label = 1;
+    comp->next_label = 0;
 
     if (scope->kind != SCOPE_FUNCTION) {
         compile_syntax_error(comp, MP_PARSE_NODE_NULL, "inline assembler must be a function");
@@ -3381,6 +3383,8 @@ mp_raw_code_t *mp_compile_to_raw_code(mp_parse_tree_t *parse_tree, qstr source_f
 
     comp->source_file = source_file;
     comp->is_repl = is_repl;
+    comp->break_label = INVALID_LABEL;
+    comp->continue_label = INVALID_LABEL;
 
     // create the module scope
     scope_t *module_scope = scope_new_and_link(comp, SCOPE_MODULE, parse_tree->root, emit_opt);