This change uses fmf subflags to guard optimizations as well as unsafe. These changes originated from D46483.
The FIXME comment is stale, and it should be removed.
The fneg does have FMF to propagate, but we're not doing that here, and that's a mistake visible in the test case.
Please add this test as an NFC preliminary commit. That way, we won't have the constant pool noise in the unrelated tests. You might also just want to put this test at the bottom of the file to avoid that noise.