[SelectionDAG] Copy FP flags when visiting a binary instruction.

Summary:
We noticed in Julia that the sequence below no longer turned into
a sequence of FMA instructions in LLVM 7+, but it did in LLVM 6.

```
    %29 = fmul contract <4 x double> %wide.load, %wide.load16
    %30 = fmul contract <4 x double> %wide.load13, %wide.load17
    %31 = fmul contract <4 x double> %wide.load14, %wide.load18
    %32 = fmul contract <4 x double> %wide.load15, %wide.load19
    %33 = fadd fast <4 x double> %vec.phi, %29
    %34 = fadd fast <4 x double> %vec.phi10, %30
    %35 = fadd fast <4 x double> %vec.phi11, %31
    %36 = fadd fast <4 x double> %vec.phi12, %32
```

Unlike Clang, Julia doesn't set the `unsafe-fp-math=true` function
attribute, but rather emits more local instruction flags.

This partially undoes https://reviews.llvm.org/D46854 and if required I can try to minimize the test further.

Reviewers: spatel, mcberg2017

Reviewed By: spatel

Subscribers: chriselrod, merge_guards_bot, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71495