Revert "[DAGCombiner] Simplifying `{si|ui}tofp` when only signbit is needed"

This reverts commit 353fbeb0. It crashes
when it encounters an UINT_TO_FP.

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp:1618 in SDValue llvm::SelectionDAG::getConstant(const ConstantInt &, const SDLoc &, EVT, bool, bool): VT.isInteger() && "Cannot create FP integer constant!"