[RISCV] Add patterns for scalable-vector fabs & fcopysign

The patterns mostly follow the scalar counterparts, save for some extra
optimizations to match the vector/scalar forms.

The patch adds a DAGCombine for ISD::FCOPYSIGN to try and reorder
ISD::FNEG around any ISD::FP_EXTEND or ISD::FP_TRUNC of the second
operand. This helps us achieve better codegen to match vfsgnjn.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D96028