[llvm] [DAG] Add computeKnownBits(FREEZE(X)) handling (PR #214095)

[Kevin Le via llvm-commits]

kneevin wrote:

I've reduced it with `llvm-reduce` and adjusted the output! I did what @arsenm suggested: codegen diff (patch vs trunk) as the interestingness test. I added three things back after using `llvm-reduce`:

1. `nounwind`
2. escape pointer; reduce nulled it and so the store was UB
3. `lshr`; partially known-bits is the specific scenario for FREEZE handler

Additionally, the reduced form now also shows the fold in the i686 RUN!

https://github.com/llvm/llvm-project/pull/214095