[NFC][AArch64] update tests for `is_fpclass` (#187336)
Hopefully this is better.
One wrinkle is that `@llvm.is.fpclass.bf16` is not currently implemented
for GI. That might be easy to add but I've not been able to figure out
where the issue is exactly so far.
I'm also not totally sure `-mattr=-fp-armv8` is equivalent to softfloat,
but some tests do suggest that they are equivalent (and looking at the
assembly, that seems right).
