[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