Fix stack restoration in to_ssa

This is similar to the functional stack passing fix in #119 but has a
smaller fix (just clobber the stacks altogether before restoring the old
values; previously we just overwrote the new variables).

Author: sampsyo

examples/test/to_ssa/if-const.out

@@ -9,7 +9,7 @@
   b.0: int = const 1;
   jmp .zexit;
 .zexit:
-  b.1: int = phi b.0 b.0 .false .true;
+  b.1: int = phi b.0 __undefined .false .true;
   a.1: int = phi __undefined a.0 .false .true;
   print a.1;
   ret;

examples/test/to_ssa/if-ssa.out

@@ -10,7 +10,7 @@
   jmp .zexit;
 .zexit:
   a.3.1: int = phi __undefined a.3.0 .left .right;
-  a.2.1: int = phi a.2.0 a.2.0 .left .right;
+  a.2.1: int = phi a.2.0 __undefined .left .right;
   a.4.0: int = phi a.2.1 a.3.1 .left .right;
   print a.4.0;
   ret;

examples/to_ssa.py

@@ -83,6 +83,7 @@ def _rename(block):
             _rename(b)
 
         # Restore stacks.
+        stack.clear()
         stack.update(old_stack)
 
     entry = list(blocks.keys())[0]