[RISCV] Add tests to demonstrate bitcasted fneg/fabs dagcombines

This target-independent code won't trigger for cases such as RV32FD where
custom SelectionDAG nodes are generated. These new tests demonstrate such
cases. Additionally, float-arith.ll was updated so that fneg.s, fsgnjn.s, and
fabs.s selection patterns are actually exercised.

llvm-svn: 352199