Removed checks and assertions used with form_Map().

The files createTaskFunctions.cpp and implementForallIntents.cpp
contained these idioms:

  form_Map(SymbolMapElem, e, ...) {
    INT_ASSERT(e->key);
    ...

and/or

  form_Map(SymbolMapElem, e, ...) {
    if (Symbol* sym = e->key) {
      ...

Since the 'form_Map' macro only yields non-NULL things by design,
the above assert and check are not needed. I am removing them.

I also replaced:      vars->put(sym, arg);
with the equivalent:  e->value = arg;
for efficiency.


These changes can also be applied to flattenFunctions.cpp.
I did not do so, to keep that code undisturbed.