[mlir] Fix infinite recursion in alias initializer

The alias initializer keeps a list of child indices around. When an alias is then marked as non-deferrable, all children are also marked non-deferrable.

This is currently done naively which leads to an infinite recursion if using mutable types or attributes containing a cycle.

This patch fixes this by adding an early return if the alias is already marked non-deferrable. Since this function is the only way to mark an alias as non-deferrable, it is guaranteed that if it is marked non-deferrable, all its children are as well, and it is not required to walk all the children.
This incidentally makes the non-deferrable marking also `O(n)` instead of `O(n^2)` (although not performance sensitive obviously).

Differential Revision: https://reviews.llvm.org/D158932