[CostModel] Add really basic support for being able to query the cost of the FNeg instruction.
This reuses the getArithmeticInstrCost, but passes dummy values of the second
The X86 costs are wrong and can be improved in a follow up. I just wanted to
stop it from reporting an unknown cost first.
Reviewers: RKSimon, spatel, andrew.w.kaylor, cameron.mcinally
Reviewed By: spatel
Subscribers: hiraditya, llvm-commits
Differential Revision: https://reviews.llvm.org/D62444