[llvm] [ConstantFolding] Add folding for [de]interleave2, insert and extract (PR #141301)

[Nikolay Panchenko via llvm-commits]

npanchen wrote:

> Can you add some constexpr tests? `getAggregateElement` may return null.

not sure how to trigger it, but just in case added bailout if it's nullptr, similar to the default code in the function

https://github.com/llvm/llvm-project/pull/141301