CSE mark_dependence instructions

This enables things like redundant load elimination and should fix the
regression after changing ManagedBuffer(Pointer) to use unsafeAddressWithOwner.

rdar://27138023

Author: aschwaighofer

lib/SIL/SILInstruction.cpp

@@ -619,6 +619,10 @@ namespace {
       return true;
     }
 
+    bool visitMarkDependenceInst(const MarkDependenceInst *RHS) {
+      return true;
+    }
+
   private:
     const SILInstruction *LHS;
   };

lib/SILOptimizer/Transforms/CSE.cpp

@@ -362,6 +362,12 @@ class HashVisitor : public SILInstructionVisitor<HashVisitor, llvm::hash_code> {
                               Operands.begin(),
                               Operands.end()));
   }
+  hash_code visitMarkDependenceInst(MarkDependenceInst *X) {
+    OperandValueArrayRef Operands(X->getAllOperands());
+    return llvm::hash_combine(
+        X->getKind(), X->getType(),
+        llvm::hash_combine_range(Operands.begin(), Operands.end()));
+  }
 };
 } // end anonymous namespace
 
@@ -680,6 +686,7 @@ bool CSE::canHandle(SILInstruction *Inst) {
     case ValueKind::BridgeObjectToWordInst:
     case ValueKind::ThinFunctionToPointerInst:
     case ValueKind::PointerToThinFunctionInst:
+    case ValueKind::MarkDependenceInst:
       return true;
     default:
       return false;

test/SILOptimizer/cse.sil

@@ -1306,3 +1306,16 @@ bb2:                                              // Preds: bb0 bb1
   return %19 : $()                                // id: %20
 }
 
+// CHECK-LABEL: sil @cse_mark_dependence
+// CHECK: mark_dependence
+// CHECK-NOT: mark_dependence
+// CHECK: return
+sil @cse_mark_dependence : $@convention(thin) (@inout Builtin.Int64, @guaranteed Builtin.NativeObject) -> (Builtin.Int64, Builtin.Int64) {
+bb0(%0 : $*Builtin.Int64, %1 : $Builtin.NativeObject):
+  %2 = mark_dependence %0 : $*Builtin.Int64 on %1 : $Builtin.NativeObject
+  %3 = mark_dependence %0 : $*Builtin.Int64 on %1 : $Builtin.NativeObject
+  %4 = load %2 : $*Builtin.Int64
+  %5 = load %3 : $*Builtin.Int64
+  %6 = tuple(%4 : $Builtin.Int64, %5 : $Builtin.Int64)
+  return %6 : $(Builtin.Int64, Builtin.Int64)
+}