[DAGCombine] visitFDIV - Use isCheaperToUseNegatedFPOps helper for (fdiv (fneg X), (fneg Y)) -> (fdiv X, Y). NFCI.
[DAGCombine] visitFDIV - Use isCheaperToUseNegatedFPOps helper for (fdiv (fneg X), (fneg Y)) -> (fdiv X, Y). NFCI.

Minor cleanup to use equivalent helper code.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@371724 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm