[llvm] [DAG] computeKnownFPClass - add ISD::SPLAT_VECTOR handling (PR #189780)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon requested changes to this pull request.

Please regenerate the tests after #189944 (get rid of any that don't use splat_vector) - yoiu might find it more convenient to just add the riscv tests to combine-is_fpclass.ll

https://github.com/llvm/llvm-project/pull/189780