AddPadding: Insert padding at the beginning of structs if necessary

This is sometimes needed now that we're removing members before adding
padding.

oi/type_graph/AddPadding.cpp

@@ -71,7 +71,9 @@ void AddPadding::visit(Class& c) {
   std::vector<Member> paddedMembers;
   paddedMembers.reserve(c.members.size());
   for (size_t i = 0; i < c.members.size(); i++) {
-    if (i >= 1) {
+    if (i == 0) {
+      addPadding(0, c.members[0].bitOffset, paddedMembers);
+    } else {
       addPadding(c.members[i - 1], c.members[i].bitOffset, paddedMembers);
     }
 

test/test_add_padding.cpp

@@ -25,6 +25,22 @@ TEST(AddPaddingTest, BetweenMembers) {
 )");
 }
 
+TEST(AddPaddingTest, AtBeginning) {
+  test(AddPadding::createPass(), R"(
+[0] Struct: MyStruct (size: 16)
+      Member: n1 (offset: 8)
+        Primitive: int64_t
+)",
+       R"(
+[0] Struct: MyStruct (size: 16)
+      Member: __oi_padding (offset: 0)
+[1]     Array: (length: 8)
+          Primitive: int8_t
+      Member: n1 (offset: 8)
+        Primitive: int64_t
+)");
+}
+
 TEST(AddPaddingTest, AtEnd) {
   auto myclass = Class{0, Class::Kind::Struct, "MyStruct", 16};
   auto myint8 = Primitive{Primitive::Kind::Int8};