[x86] remove 'nan' strings from copysign assertions; NFC

Preemptively scrubbing these to avoid a bot fail as in PR30443:
https://llvm.org/bugs/show_bug.cgi?id=30443

I'm nearly done with a patch to fix these cases, so not trying very
hard to do better for the temporary win. 

I plan to use better checks than what the script produces for the vectorized cases.

llvm-svn: 283072