[llvm] [Semilattice] Introduce for dataflow analysis with KnownBits (PR #177616)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> Do a fixpoint-iteration, until the root's KnownBits doesn't change.

IIRC, the height of the lattice may be too high, and we may take many iterations to reach the fix point. That is why we have a `NumRangeExtensions` field in `ValueLatticeElement`. See section 6.2 of https://cs.au.dk/~amoeller/spa/spa.pdf

https://github.com/llvm/llvm-project/pull/177616