ir: Tweak monomorphization to instantiate recursively after inserting to the replacements map.

That way the check to prevent double-instantiation also works to prevent
infinite recursion.

tests/rust/infinite_recursion_typedef_monomorph.rs

@@ -0,0 +1,2 @@
+pub type TryVec<T> = fallible_collections::TryVec<T>;
+pub type TryString = fallible_collections::TryVec<u8>;