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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

> 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

Yes, this was just the initial setup work - I'll create additional sub-issues to #137943 shortly. Cheers.

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