[3.15] gh-151112: Move an assert that may fail in cfg_builder_check (GH-151153) (#151313)

(cherry picked from commit 2d33810)

Co-authored-by: Stan Ulbrych <stan@python.org>

Author: miss-islington

Python/flowgraph.c

@@ -403,7 +403,6 @@ cfg_builder_maybe_start_new_block(cfg_builder *g)
 static bool
 cfg_builder_check(cfg_builder *g)
 {
-    assert(g->g_entryblock->b_iused > 0);
     for (basicblock *block = g->g_block_list; block != NULL; block = block->b_list) {
         assert(!_PyMem_IsPtrFreed(block));
         if (block->b_instr != NULL) {
@@ -3756,6 +3755,7 @@ _PyCfg_OptimizeCodeUnit(cfg_builder *g, PyObject *consts, PyObject *const_cache,
                         int nlocals, int nparams, int firstlineno)
 {
     assert(cfg_builder_check(g));
+    assert(g->g_entryblock->b_iused > 0);
     /** Preprocessing **/
     /* Map labels to targets and mark exception handlers */
     RETURN_IF_ERROR(translate_jump_labels_to_targets(g->g_entryblock));