Infer VectorInstrContext::BinaryOp in TTI::getVectorInstrContextHint

From the discussion in https://github.com/llvm/llvm-project/pull/209387#discussion_r3594816822

We can use this to then write tests via `opt -p 'print'`, instead of through the loop vectorizer tests.

Start with the discussion in PR 209387, especially the referenced comment, and trace TTI::getVectorInstrContextHint and VectorInstrContext::BinaryOp. Use `opt -p 'print'` to inspect the relevant output; done means the binary-operation context is inferred and covered by cost-model tests rather than loop vectorizer tests.

Written by the indexing model from the issue text.