Strengthened CallExpr::verify() and used this in PRIM_TYPE_INIT case in removeRandomPrimitive().

In CallExpr::verify(), added a clause to ensure that every CallExpr has a valid
parentExpr.  Also added a test specific to PRIM_TYPE_INIT, that that parentExpr is a
CallExpr.  Both of these added clauses pass a full --verify test run.

Added comments in the clause for PRIM_TYPE_INIT in removeRandomPrimitive(), saying that we
don't have to test either of these invariants (call->parentExpr != NULL &&
toCallExpr(call->parentExpr) != NULL), because CallExpr::verify() does it for us.

There is a general caveat on this assumption, since verify only operates between passes.
If the current pass violates the invariant, one or both assumptions may fail.  A good
debug technique for this would be to run verify() on the suspect AST node.