[InstSimplify] Rename SimplifyFPUnOp and SimplifyFPBinOp

Summary:
SimplifyFPBinOp is a variant of SimplifyBinOp that lets you specify
fast math flags, but the name is misleading because both functions
can simplify both FP and non-FP ops. Instead, overload SimplifyBinOp
so that you can optionally specify fast math flags.

Likewise for SimplifyFPUnOp.

Reviewers: spatel

Reviewed By: spatel

Subscribers: xbolva00, cameron.mcinally, eraman, hiraditya, haicheng, zzheng, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64902

llvm-svn: 366902