[CostModel] Add really basic support for being able to query the cost of the FNeg instruction.

Summary:
This reuses the getArithmeticInstrCost, but passes dummy values of the second
operand flags.

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62444

llvm-svn: 361788