[InlineCost] Don't add the soft float function call cost for the fneg idiom, fsub -0.0, %x

Summary: Fneg can be implemented with an xor rather than a function call so we don't need to add the function call overhead. This was pointed out in D62699

Reviewers: efriedma, cameron.mcinally

Reviewed By: efriedma

Subscribers: javed.absar, eraman, hiraditya, haicheng, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62747

llvm-svn: 362304