[opt] Fix bug opt::InstructionBuilder::AddVariable (#6007)

The storage class operand was being added with the wrong
operand type.  It was assumed to be an ID.  Then trying
to analyze the new variable declaration instruction's
defs and uses could get confused and assert out.

Added a test for this.

source/opt/ir_builder.h

diff --git a/source/opt/ir_builder.h b/source/opt/ir_builder.h
index f3e0afc..3d732cc 100644
--- a/source/opt/ir_builder.h
+++ b/source/opt/ir_builder.h
@@ -516,7 +516,7 @@
 
   Instruction* AddVariable(uint32_t type_id, uint32_t storage_class) {
     std::vector<Operand> operands;
-    operands.push_back({SPV_OPERAND_TYPE_ID, {storage_class}});
+    operands.push_back({SPV_OPERAND_TYPE_STORAGE_CLASS, {storage_class}});
     std::unique_ptr<Instruction> new_inst(
         new Instruction(GetContext(), spv::Op::OpVariable, type_id,
                         GetContext()->TakeNextId(), operands));

test/opt/ir_builder.cpp

diff --git a/test/opt/ir_builder.cpp b/test/opt/ir_builder.cpp
index f0cfc18..15d31d9 100644
--- a/test/opt/ir_builder.cpp
+++ b/test/opt/ir_builder.cpp
@@ -256,6 +256,38 @@
   Match(text, context.get());
 }
 
+TEST_F(IRBuilderTest, AddVariable) {
+  // Use Private beacuse its' enun is 7 which is higher
+  // than the ID limit.
+  const std::string text = R"(
+; CHECK: [[uint:%\w+]] = OpTypeInt 32 0
+; CHECK: [[ptr:%\w+]] = OpTypePointer Private [[uint]]
+; CHECK: [[var:%\w+]] = OpVariable [[ptr]] Private
+; CHECK: OpTypeFloat
+OpCapability Kernel
+OpCapability VectorComputeINTEL
+OpCapability Linkage
+OpExtension "SPV_INTEL_vector_compute"
+OpMemoryModel Logical OpenCL
+%1 = OpTypeInt 32 0
+%2 = OpTypePointer Private %1
+%3 = OpTypeFloat 32
+)";
+
+  std::unique_ptr<IRContext> context =
+      BuildModule(SPV_ENV_UNIVERSAL_1_2, nullptr, text);
+  EXPECT_NE(nullptr, context) << text;
+
+  auto* float_ty = context->get_def_use_mgr()->GetDef(3);
+  InstructionBuilder builder(context.get(), float_ty);
+  auto* var = builder.AddVariable(2u, uint32_t(spv::StorageClass::Private));
+  EXPECT_NE(nullptr, var);
+
+  context->get_def_use_mgr()->AnalyzeInstDefUse(var);  // should not assert
+
+  Match(text, context.get());
+}
+
 TEST_F(IRBuilderTest, AddCompositeConstruct) {
   const std::string text = R"(
 ; CHECK: [[uint:%\w+]] = OpTypeInt