[DAGCombiner] prevent unsafe reassociation of FP ops
[DAGCombiner] prevent unsafe reassociation of FP ops

There are no FP callers of DAGCombiner::reassociateOps() currently,
but we can add a fast-math check to make sure this API is not being
misused.

This was noted as a potential risk (and that risk might increase) with:
D62191

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@361268 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm