[RISCV] Move fli selection in RISCVISelDAGToDAG.cpp. NFC

We custom isel for ConstantFP that has higher priority than isel
patterns. We were previously detecting valid FP constants for fli
to early exit from the custom code. This detection called
getLoadFPImm. Then we would run the isel patterns which would call
getLoadFPImm a second time.

With a little bit more code we can directly select the fli instruction
in the custom handler and avoid a second call.

Remove the incorrect mayRaiseFPException flag from the FLI instructions.

Reviewed By: joshua-arch1

Differential Revision: https://reviews.llvm.org/D146093