[llvm] [DAG] Add initial version of SelectionDAG::computeKnownFPClass (PR #188790)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> @RKSimon Thanks for the review and guidance on this. It's been a great learning experience working on `computeKnownFPClass`. I'll keep an eye on `computeKnownFPClass`, and I'm happy to pick up related issue as it comes~

This surely could use follow ups to actually implement all of the opcodes, and plug in the use. This should eventually retire isKnownNeverNaN. 

Most of the logic work is taken care of already in Support's KnownFPClass 

https://github.com/llvm/llvm-project/pull/188790