[Docs] Add LangRef documentation for freeze instruction
Summary:
- Describe the new freeze instruction
- Make it explicit that branch on undef/poison is UB
Reviewers: chandlerc, majnemer, efriedma, nikic, reames, jdoerfert, lebedev.ri, regehr
Subscribers: fhahn, bollu, lebedev.ri, delcypher, spatel, filcab, llvm-commits, aqjune
Differential Revision: https://reviews.llvm.org/D29121