py: Fix 2 bugs in native emitter: jump_or_pop and stack settling.

Addresses issue #838.

tests/basics/ifcond.py

@@ -0,0 +1,35 @@
+# test if conditions which are optimised by the compiler
+
+f2 = 0
+
+def f(t1, t2, f1):
+    if False:
+        print(1)
+    if True:
+        print(1)
+    if ():
+        print(1)
+    if (1,):
+        print(1)
+    if (1, 2):
+        print(1)
+    if t1 and t2:
+        print(1)
+    if (t1 and t2): # parsed differently to above
+        print(1)
+    if not (t1 and f1):
+        print(1)
+    if t1 or t2:
+        print(1)
+    if (t1 or t2): # parse differently to above
+        print(1)
+    if f1 or t1:
+        print(1)
+    if not (f1 or f2):
+        print(1)
+    if t1 and f1 or t1 and t2:
+        print(1)
+    if (f1 or t1) and (f2 or t2):
+        print(1)
+
+f(True, 1, False)