py/compile: Simplify compilation of comprehension iterators.

Saves 88 bytes on Thumb2, and 200 bytes on x86-64 archs.

py/compile.c

@@ -2660,7 +2660,14 @@ STATIC void compile_scope_func_annotations(compiler_t *comp, mp_parse_node_t pn)
 }
 #endif // MICROPY_EMIT_NATIVE
 
-STATIC void compile_scope_comp_iter(compiler_t *comp, mp_parse_node_t pn_iter, mp_parse_node_t pn_inner_expr, int l_top, int for_depth) {
+STATIC void compile_scope_comp_iter(compiler_t *comp, mp_parse_node_struct_t *pns_comp_for, mp_parse_node_t pn_inner_expr, int for_depth) {
+    uint l_top = comp_next_label(comp);
+    uint l_end = comp_next_label(comp);
+    EMIT_ARG(label_assign, l_top);
+    EMIT_ARG(for_iter, l_end);
+    c_assign(comp, pns_comp_for->nodes[0], ASSIGN_STORE);
+    mp_parse_node_t pn_iter = pns_comp_for->nodes[2];
+
     tail_recursion:
     if (MP_PARSE_NODE_IS_NULL(pn_iter)) {
         // no more nested if/for; compile inner expression
@@ -2688,17 +2695,13 @@ STATIC void compile_scope_comp_iter(compiler_t *comp, mp_parse_node_t pn_iter, m
         // for loop
         mp_parse_node_struct_t *pns_comp_for2 = (mp_parse_node_struct_t*)pn_iter;
         compile_node(comp, pns_comp_for2->nodes[1]);
-        uint l_end2 = comp_next_label(comp);
-        uint l_top2 = comp_next_label(comp);
         EMIT(get_iter);
-        EMIT_ARG(label_assign, l_top2);
-        EMIT_ARG(for_iter, l_end2);
-        c_assign(comp, pns_comp_for2->nodes[0], ASSIGN_STORE);
-        compile_scope_comp_iter(comp, pns_comp_for2->nodes[2], pn_inner_expr, l_top2, for_depth + 1);
-        EMIT_ARG(jump, l_top2);
-        EMIT_ARG(label_assign, l_end2);
-        EMIT(for_iter_end);
+        compile_scope_comp_iter(comp, pns_comp_for2, pn_inner_expr, for_depth + 1);
     }
+
+    EMIT_ARG(jump, l_top);
+    EMIT_ARG(label_assign, l_end);
+    EMIT(for_iter_end);
 }
 
 STATIC void check_for_doc_string(compiler_t *comp, mp_parse_node_t pn) {
@@ -2862,16 +2865,8 @@ STATIC void compile_scope(compiler_t *comp, scope_t *scope, pass_kind_t pass) {
         #endif
         }
 
-        uint l_end = comp_next_label(comp);
-        uint l_top = comp_next_label(comp);
         compile_load_id(comp, qstr_arg);
-        EMIT_ARG(label_assign, l_top);
-        EMIT_ARG(for_iter, l_end);
-        c_assign(comp, pns_comp_for->nodes[0], ASSIGN_STORE);
-        compile_scope_comp_iter(comp, pns_comp_for->nodes[2], pns->nodes[0], l_top, 0);
-        EMIT_ARG(jump, l_top);
-        EMIT_ARG(label_assign, l_end);
-        EMIT(for_iter_end);
+        compile_scope_comp_iter(comp, pns_comp_for, pns->nodes[0], 0);
 
         if (scope->kind == SCOPE_GEN_EXPR) {
             EMIT_ARG(load_const_tok, MP_TOKEN_KW_NONE);