[llvm] [RISCV] Undo fneg (fmul x, y) -> fmul x, (fneg y) transform (PR #157388)

[Luke Lau via llvm-commits]

lukel97 wrote:

> > This also causes some more fnegs to be sunk after a bitcast to integer, so they're now done as xor.
> 
> Why is that, if the transform is for a single use of fneg in fmul?

We go from `bitcast (fmul (fneg)) -> bitcast (fneg (fmul))`, and then there's a generic DAGCombine that does `(bitconvert (fneg x)) -> (xor (bitconvert x), signbit)` in `DAGCombiner::visitBITCAST`

https://github.com/llvm/llvm-project/pull/157388