py/compile: Optimize code generated for "except Exc as var:".

When such a clause is used (unlike just "except Exc:"), CPython wraps
exception handler body in try-finally block, and in the finally part,
executes "var = None; del var". The initial assignments appears to be
superfluous with the successive "del". It's there only to cover a case
when exeception handler would contain "del var" itself. Thus, compiler-
generated code would recreate the var and then delete again, to avoid
any spurious exception. Such a code would be rare, so avoid generating
assignment unless MICROPY_CPYTHON_COMPAT is defined.

Change-Id: I9a71821e394f87ff4153ae7c016e7d300147c429

py/compile.c

@@ -1595,8 +1595,16 @@ STATIC void compile_try_except(compiler_t *comp, mp_parse_node_t pn_body, int n_
         if (qstr_exception_local != 0) {
             EMIT_ARG(load_const_tok, MP_TOKEN_KW_NONE);
             EMIT_ARG(label_assign, l3);
+            #if MICROPY_CPYTHON_COMPAT
+            // This follows CPython, and the only reason for this store is
+            // to allow code like:
+            // except Exc as foo:
+            //     del foo
+            // That's quite a rare case, so save 2 bytecode bytes unless
+            // full CPython compatibility is requested.
             EMIT_ARG(load_const_tok, MP_TOKEN_KW_NONE);
             compile_store_id(comp, qstr_exception_local);
+            #endif
             compile_delete_id(comp, qstr_exception_local);
 
             compile_decrease_except_level(comp);