Correctly increment/decrement pointers to large (>=64KiB) types.

Previously, the logic for this was incorrect and would lead to a null pointer dereference in the compiler. In most cases the generated code would not actually change the pointer.

The following program demonstrates the issue:

#include <stdio.h>
#pragma memorymodel 1
typedef char bigarray[0x20000];
bigarray big[5];
int main(void) {
        bigarray *p = big;
        p++;
        printf("%p %p\n", (void*)big, (void*)p);
}

Expression.pas

@@ -2535,7 +2535,7 @@ var
                   Gen0(pc_adl)
                else
                   Gen0(pc_sbl);
-               with tree^.left^.id^ do
+               with tree^.id^ do
                   case storage of
                      stackFrame, parameter:
                         Gen2t(pc_cop, lln, 0, cgULong);