Remove BinaryOperator::CreateFNeg

Use UnaryOperator::CreateFNeg instead.

Summary:
With the introduction of the native fneg instruction, the
fsub -0.0, %x idiom is obsolete. This patch makes LLVM
emit fneg instead of the idiom in all places.

Reviewed By: cameron.mcinally

Differential Revision: https://reviews.llvm.org/D75130