If fneg lowering for fsub -0.0, x fails we currently fall back to treating it as an fsub. This has different behavior for nans than the xor with sign bit trick we normally try to do. On X86, the xor trick for double fails fast-isel in 32-bit mode with sse2 due to 64 bit integer types not being available. With -O2 we would always use an xorpd for this case. If we use subsd, this creates an observable behavior difference between -O0 and -O2. So fall back to SelectionDAG if we can't fast-isel it, that way SelectionDAG will use the xorpd.
I believe this patch is restoring the behavior prior to r345295 from last October. This was missed then because our fast isel case in 32-bit mode aborted fast-isel earlier for another reason. But I've added new tests to cover that.