[llvm] [DAG] Add SDPatternMatch m_ZExtOrSelf/m_SExtOrSelf/m_AExtOrSelf/m_TruncOrSelf matchers (PR #85480)

[Marc Auberer via llvm-commits]

marcauberer wrote:

@RKSimon @mshockwave this is now ready for review.
I am not entirely sure how I would add negative tests, since an expression does always match with e.g. `m_ZExtOrSelf()`, because it is always "self". Maybe take a larger expression from InstCombine, that uses the InstCombine `m_ZExtOrSelf()` and modify it for the DAG unit test? What do you think?

https://github.com/llvm/llvm-project/pull/85480