[llvm] [ValueTracking] Bail out on x86_fp80 when computing fpclass with knownbits (PR #130477)

[Matt Arsenault via llvm-commits]

arsenm wrote:

Probably should remove the isIEEE from APFloat. It has repeatedly caused confusion and it only points out an irrelevant APFloat implementation detail 

https://github.com/llvm/llvm-project/pull/130477