[llvm] SelectionDAG: Use qNaN constant if FCANONICALIZE not LegalOrCustom (PR #104564)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > You cannot do that. A regular fmul does not have the same restrictions as fcanonicalize. A later canonicalize dropping combine could lose the required semantics
> 
> So we'd need ISD::STRICT_FMUL or is that still not enough?

That depends on how acceptable it is to introduce strict FP nodes in a non-strict function. The IR enforces that's not OK, but the DAG is more permissive (not necessarily by design?). In principle it should work. 

https://github.com/llvm/llvm-project/pull/104564