Python: Fix bad magic in controls/2

The changes to `ModificationOfParameterWithDefault.ql` and the use of
`ConditionBlock::controls` therein caused the `BasicBlock` argument to
get magicked in, resulting in the following antijoin for the `forall`:

```
[2021-10-04 12:07:46] (108s) Tuple counts for GuardedControlFlow::ConditionBlock::controls_dispred#fbf#antijoin_rhs/5@d84e94 after 1m44s:
201222345 ~7%     {5} r1 = JOIN GuardedControlFlow::ConditionBlock::controls_dispred#fbf#shared#2 WITH Flow::BasicBlock::getASuccessor_dispred#ff_10#join_rhs ON FIRST 1 OUTPUT Lhs.0 'arg1', Rhs.1 'arg4', Lhs.1 'arg0', Lhs.2 'arg2', Lhs.3 'arg3'
200599933 ~4%     {5} r2 = JOIN r1 WITH Flow::BasicBlock::dominates#ff ON FIRST 2 OUTPUT Lhs.2 'arg0', Lhs.0 'arg1', Lhs.3 'arg2', Lhs.4 'arg3', Lhs.1 'arg4'

0         ~0%     {4} r3 = JOIN GuardedControlFlow::ConditionBlock::controls_dispred#fbf#shared#1 WITH GuardedControlFlow::ConditionBlock#class#f ON FIRST 1 OUTPUT Lhs.0 'arg3', Lhs.2 'arg1', Lhs.1 'arg0', false

0         ~0%     {4} r4 = JOIN GuardedControlFlow::ConditionBlock::controls_dispred#fbf#shared WITH GuardedControlFlow::ConditionBlock#class#f ON FIRST 1 OUTPUT Lhs.0 'arg3', Lhs.2 'arg1', Lhs.1 'arg0', true

0         ~0%     {4} r5 = r3 UNION r4
0         ~0%     {5} r6 = JOIN r5 WITH Flow::BasicBlock::getASuccessor_dispred#ff ON FIRST 2 OUTPUT Lhs.2 'arg0', Lhs.1 'arg1', Lhs.3 'arg2', Lhs.0 'arg3', Rhs.0

200599933 ~4%     {5} r7 = r2 UNION r6
                return r7
```
(cancelled)

I observed that quick-eval'ing the `controls` predicate exhibit no such
bad join order (and terminated quickly) which lead me to conclude that
this was a case of bad magic.

Adding the `pragma[nomagic]` resulted in a return to the previous
performance.

Author: tausbn

python/ql/lib/semmle/python/GuardedControlFlow.qll

@@ -9,6 +9,7 @@ class ConditionBlock extends BasicBlock {
   }
 
   /** Basic blocks controlled by this condition, i.e. those BBs for which the condition is testIsTrue */
+  pragma[nomagic]
   predicate controls(BasicBlock controlled, boolean testIsTrue) {
     /*
      * For this block to control the block 'controlled' with 'testIsTrue' the following must be true: