[RISCV] Add another potential combine to {double,float}-bitmanip-dagcombines.ll

(fcopysign a, (fneg b)) will be expanded to bitwise operations by
DAGTypeLegalizer::SoftenFloatRes_FCOPYSIGN if the floating point type isn't
legal. Arguably it might be worth doing a combine even if it is legal.

llvm-svn: 352240