[llvm] Revert "ValueTracking: Improve handling of fadd in computeKnownFPClass." (PR #174290)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> hack in enabling flushing for AArch64.

Which won't help anyway since it doesn't have the separate input and output mode 


https://github.com/llvm/llvm-project/pull/174290