Disqualify graph traversal for loops

[NathanBaulch]

The following is a common pattern for traversing linked lists or tree node ancestors:
for n := first(); n != nil; n = n.next() {
There's no way to determine the iteration count statically for for loops that advance the loop variable with a function call, so these need to be disqualified.

At some point I'd like to enable the --forloops option by default once we can reliably handle these common for loop patterns.

[alexkohler]

At some point I'd like to enable the --forloops option by default once we can reliably handle these common for loop patterns

Agree! Thanks again for all of the PRs + improving prealloc 🚀