[IR] Add Freeze instruction

Summary:

• Define Instruction::Freeze, let it be UnaryOperator
• Add support for freeze to LLLexer/LLParser/BitcodeReader/BitcodeWriter The format is %x = freeze <ty> %v
• Add support for freeze instruction to llvm-c interface.
• Add m_Freeze in PatternMatch.
• Erase freeze when lowering IR to SelDag.

Reviewers: deadalnix, hfinkel, efriedma, lebedev.ri, nlopes, jdoerfert, regehr, filcab, delcypher, whitequark

Reviewed By: lebedev.ri, jdoerfert

Subscribers: jfb, kristof.beyls, hiraditya, lebedev.ri, steven_wu, dexonsmith, xbolva00, delcypher, spatel, regehr, trentxintong, vsk, filcab, nlopes, mehdi_amini, deadalnix, llvm-commits

Differential Revision: https://reviews.llvm.org/D29011