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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Hi @arsenm can you clarify my query? Is this PR trying to achieve the same goal as mine, If not I can keep preparing my patch, which I plan to submit in a few days.

It is not. Your PR would be a requirement for this to be usable on x86. X86 (and all targets) need to implement canonicalize, which cannot be done in terms of other generic floating point ops. It needs to be directly selected (or we need to rewrite the rules for DAG nodes to be different from IR, audit all the combines, and then suffer from having different rules from the IR)

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