[llvm] DAG: Lower single infinity is.fpclass tests to fcmp (PR #100380)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon approved this pull request.

LGTM (although it'd have been nice if we could check the comparison value as well instead of it being lost behind a constant pool load - not your problem though).

https://github.com/llvm/llvm-project/pull/100380