[DAG] isKnownNeverNaN - fallback to computeKnownFPClass check (#189476)
Remove ConstantFPSDNode handling from isKnownNeverNaN and fallback to
using computeKnownFPClass if there are no opcode matches in
isKnownNeverNaN
The test check changes are due to isKnownNeverNaN not handling
UNDEF/POISON but computeKnownFPClass does (POISON in particular now
returns isKnownNeverNaN == true, preventing a ISD::FCANONICALIZE call in
expandFMINNUM_FMAXNUM).