[Snippets] Fixed insertion position iterators

[a-sidorova]

Details:

• Fixed Buffer insertion position: potential_consumers is unsorted by linear IR set. We have to find first expr in Linear IR from the set to insert Buffer before all consumers
• Added OPENVINO_ASSERT for all std::find if needed

Tickets:

• N/A

[a-sidorova]

@IvanNovoselov could you take a look one more time please? Thank you!