[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

1 files changed, 5 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index 0e6e7027e0c..914edad0b8b 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -10592,3 +10592,8 @@ void SelectionDAGBuilder::visitSwitch(const SwitchInst &SI) {
     lowerWorkItem(W, SI.getCondition(), SwitchMBB, DefaultMBB);
   }
 }
+
+void SelectionDAGBuilder::visitFreeze(const User &I) {
+  SDValue N = getValue(I.getOperand(0));
+  setValue(&I, N);
+}