[llvm] [DAG] computeKnownFPClass - add ISD::EXTRACT_SUBVECTOR/INSERT_SUBVECTOR handling. (PR #190378)

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Xylecrack wrote:

I have attempted to test it with `fabs`. However i read this [comment](https://github.com/llvm/llvm-project/issues/189483#issuecomment-4179761430) about bitcasts. So iam not sure if iam headed in the right direction..

https://github.com/llvm/llvm-project/pull/190378