copy elision: runtime struct init

```zig
export fn entry() void {
    var x = Foo.{
        .x = 1,
        .y = bar(),
    };
}
```

```llvm
define void @entry() #2 !dbg !41 {
Entry:
  %x = alloca %Foo, align 4
  %0 = getelementptr inbounds %Foo, %Foo* %x, i32 0, i32 0, !dbg !56
  store i32 1, i32* %0, align 4, !dbg !57
  %1 = getelementptr inbounds %Foo, %Foo* %x, i32 0, i32 1, !dbg !58
  call fastcc void @bar(%Bar* sret %1), !dbg !59
  ret void, !dbg !60
}
```

Author: andrewrk

src/all_types.hpp

@@ -436,7 +436,7 @@ enum NodeType {
     NodeTypeArrayType,
     NodeTypeErrorType,
     NodeTypeIfErrorExpr,
-    NodeTypeTestExpr,
+    NodeTypeIfOptional,
     NodeTypeErrorSetDecl,
     NodeTypeCancel,
     NodeTypeResume,
@@ -2370,6 +2370,7 @@ struct IrInstructionFieldPtr {
     IrInstruction *container_ptr;
     Buf *field_name_buffer;
     IrInstruction *field_name_expr;
+    IrInstruction *container_type;
     bool is_const;
 };
 

src/analyze.cpp

@@ -3517,7 +3517,7 @@ void scan_decls(CodeGen *g, ScopeDecls *decls_scope, AstNode *node) {
         case NodeTypeArrayType:
         case NodeTypeErrorType:
         case NodeTypeIfErrorExpr:
-        case NodeTypeTestExpr:
+        case NodeTypeIfOptional:
         case NodeTypeErrorSetDecl:
         case NodeTypeCancel:
         case NodeTypeResume:

src/ast_render.cpp

@@ -235,8 +235,8 @@ static const char *node_type_str(NodeType node_type) {
             return "ErrorType";
         case NodeTypeIfErrorExpr:
             return "IfErrorExpr";
-        case NodeTypeTestExpr:
-            return "TestExpr";
+        case NodeTypeIfOptional:
+            return "IfOptional";
         case NodeTypeErrorSetDecl:
             return "ErrorSetDecl";
         case NodeTypeCancel:
@@ -938,7 +938,7 @@ static void render_node_extra(AstRender *ar, AstNode *node, bool grouped) {
                 }
                 break;
             }
-        case NodeTypeTestExpr:
+        case NodeTypeIfOptional:
             {
                 fprintf(ar->f, "if (");
                 render_node_grouped(ar, node->data.test_expr.target_node);