Handle 3.7+ "else" branch removal...

As seen in _cmp() of python3.8/distutils/version.py with optimization -O2

test/add-test.py

@@ -19,6 +19,7 @@ for path in py_source:
     else:
         cfile = "bytecode_%s%s/%s" % (version, suffix, short) + "c"
     print("byte-compiling %s to %s" % (path, cfile))
-    py_compile.compile(path, cfile)
+    optimize = 2
+    py_compile.compile(path, cfile, optimize=optimize)
     if isinstance(version, str) or version >= (2, 6, 0):
         os.system("../bin/uncompyle6 -a -T %s" % cfile)

test/simple_source/bug37/03_else_removal.py

@@ -0,0 +1,12 @@
+# From python3.8/distutils/version.py with optimization -O2
+# The bug was that the other "else" constant propagated removed.
+
+# NOTE: this program needs to be compile with optimization
+def _cmp (b, c):
+    if b:
+        if c:
+            return 0
+        else:
+            return 1
+    else:
+        assert False, "never get here"

uncompyle6/parsers/parse37.py

@@ -94,6 +94,9 @@ class Python37Parser(Python37BaseParser):
         else_suitec ::= c_stmts
         else_suitec ::= returns
 
+        else_suite_opt ::= else_suite
+        else_suite_opt ::= pass
+
         stmt ::= classdef
         stmt ::= call_stmt
 
@@ -635,6 +638,12 @@ class Python37Parser(Python37BaseParser):
         if_exp37                   ::= expr expr jf_cfs expr COME_FROM
         jf_cfs                     ::= JUMP_FORWARD _come_froms
         ifelsestmt                 ::= testexpr c_stmts_opt jf_cfs else_suite opt_come_from_except
+
+        # This is probably more realistically an "ifstmt" (with a null else)
+        # see _cmp() of python3.8/distutils/__pycache__/version.cpython-38.opt-1.pyc
+        ifelsestmt                 ::= testexpr stmts jf_cfs else_suite_opt opt_come_from_except
+
+
         expr_pjit                  ::= expr POP_JUMP_IF_TRUE
         expr_jit                   ::= expr JUMP_IF_TRUE
         expr_jt                    ::= expr jmp_true