Treat is_unique as a read in LoadBorrowImmutabilityChecker

Author: meg-gupta

lib/SIL/Verifier/LoadBorrowImmutabilityChecker.cpp

@@ -97,6 +97,7 @@ bool GatherWritesVisitor::visitUse(Operand *op, AccessUseType useTy) {
   case SILInstructionKind::DeallocBoxInst:
   case SILInstructionKind::WitnessMethodInst:
   case SILInstructionKind::ExistentialMetatypeInst:
+  case SILInstructionKind::IsUniqueInst:
     return true;
 
   // Known writes...

test/SIL/ownership-verifier/load_borrow_invalidation_test.sil

@@ -395,3 +395,17 @@ bb0(%0 : @owned $Builtin.NativeObject):
   %9999 = tuple()
   return %9999 : $()
 }
+
+struct Bool {
+  var _value: Builtin.Int1
+}
+
+sil [ossa] @test_withisunique : $@convention(method) <Element> (@inout Builtin.NativeObject) -> Bool {
+bb0(%0 : $*Builtin.NativeObject):
+  %1 = is_unique %0 : $*Builtin.NativeObject    
+  %2 = struct $Bool (%1 : $Builtin.Int1)        
+  %3 = load_borrow %0 : $*Builtin.NativeObject 
+  end_borrow %3 : $Builtin.NativeObject
+  return %2 : $Bool
+}
+