This reverts commit 9642aadf7064192164d1687378d28d6bda1978c9.
Since elem() only works on primitive arrays anyway, we don't have to do
the isArrayRoot() check at all.
As a way of writing atIndex(I).deref<T>(), which creates an intermediate
Pointer, which in turn adds (and removes) that pointer from the pointer
list of the Block. This way we can avoid that.
Note that PointerUnion::dyn_cast has been soft deprecated in
PointerUnion.h:
// FIXME: Replace the uses of is(), get() and dyn_cast() with
// isa<T>, cast<T> and the llvm::dyn_cast<T>
Literal migration would result in dyn_cast_if_present (see the
definition of PointerUnion::dyn_cast), but this patch uses dyn_cast
because we expect Source to be nonnull.
