GCBI: remove assertion on Dispatcher != nullptr

This assertion was executed in the `runOnModule` method of the GCBI
pass, but it is too strict.

Asserting that Dispatcher != nullptr during the execution of the pass
prevents the pass from successfully running on a Module without a
Dispatcher.
This is unfortunate, because some passes that depend on GCBI in revng-c
actually do that. They do so because they are not really interested in
the Dispatcher at all. They only use GCBI to access information on the
Stack Pointer.

In fact, the check for Dispatcher being != from nullptr is already
performed also in the `dispatcher()` method which is the sane thing to
do. This commit removes the assertion from the `runOnModule` method and
leaves it in the `dispatcher` method.
This allows to run the pass successfully even on a Module without the
Dispatcher, while not changing the contract of this pass (i.e. that when
you call the `dispatcher` method you get a nonnull pointer).

lib/BasicAnalyses/GeneratedCodeBasicInfo.cpp

@@ -100,8 +100,6 @@ bool GeneratedCodeBasicInfo::runOnModule(Module &M) {
     }
   }
 
-  revng_assert(Dispatcher != nullptr);
-
   if (auto *NamedMD = M.getNamedMetadata("revng.csv")) {
     auto *Tuple = cast<MDTuple>(NamedMD->getOperand(0));
     for (const MDOperand &Operand : Tuple->operands()) {