[RISCV] Move all isel cases for ISD::ConstantFP into RISCVDAGToDAGISel::Select.

After D142953, non-zero cases were handled in RISCVDAGToDAGISel::Select
and zeros were handled with isel patterns. The zeros cases are
sufficiently similar to zero that we might as well handle them all
together. We already needed to detect the cases to skip out to
tablegen.