[X86] Add more test cases for fast-isel handling of fneg.

The fneg double case is falling back to a subsd in 32-bit mode if you write a test that doesn't trigger a fast-isel abort on the return value.

The subsd lowering has different behavior with respect to nans than using an xor. This is inconsisent with what we would do in SelectionDAG
and can lead to differences between -O0 and -O2.

llvm-svn: 360088