[llvm] Reland "[ValueTracking] Compute knownbits from known fp classes" (PR #92084)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> Hi @dtcxzyw The following starts crashing with this patch:
> 
> ```
> opt -passes="instcombine" bbi-95530.ll -o /dev/null
> ```
> 
> It fails with
> 
> ```
> opt: ../lib/Analysis/ValueTracking.cpp:1152: void computeKnownBitsFromOperator(const llvm::Operator *, const llvm::APInt &, llvm::KnownBits &, unsigned int, const llvm::SimplifyQuery &): Assertion `!Known.hasConflict() && "Bits known to be one AND zero?"' failed.
> ```
> 
> [bbi-95530.ll.gz](https://github.com/llvm/llvm-project/files/15330187/bbi-95530.ll.gz)

It is weird that `computeKnownFPClass` returns `(none)`. I will post a fix.

BTW, I don't think we need to revert this patch.


https://github.com/llvm/llvm-project/pull/92084