split IrInstruction into IrInst, IrInstSrc, IrInstGen

This makes it so that less memory is used for IR instructions, as well
as catching bugs when one expected one kind of instruction and received
the other.

src/parser.cpp

@@ -147,7 +147,7 @@ static void ast_invalid_token_error(ParseContext *pc, Token *token) {
 }
 
 static AstNode *ast_create_node_no_line_info(ParseContext *pc, NodeType type) {
-    AstNode *node = allocate<AstNode>(1);
+    AstNode *node = allocate<AstNode>(1, "AstNode");
     node->type = type;
     node->owner = pc->owner;
     return node;