[llvm] [ConstantFolding] Fix llvm.is.fpclass folding of x87 pseudo-NaN (PR #212011)

[Milin Bhade via llvm-commits]

mbhade-amd wrote:

> > > By default? O_o (support for these formats was - at least formerly - supported by setting the correct mode flags)
> > 
> > 
> > Yes, it is handled this way by default.
> 
> Now I'm wondering if I actually did that properly when I implemented all that a few years back :-O

Gentle Ping @ojhunt @jcranmer-intel @dtcxzyw 

https://github.com/llvm/llvm-project/pull/212011