Guard for T_OBJECT at compile time (ruby#53)

Previously this could crash on Nokogiri when JITing the getivar
instruction because we would attempt to treat Nokogiri::XML::Document's
T_DATA as a T_OBJECT in calling rb_iv_index_tbl_lookup.

This commit also checks for T_OBJECT at compile time and emits the
rb_ivar_get fallback in that case.

Co-authored-by: HParker <HParker@github.com>
Co-authored-by: Dinah Shi <dinahshi@github.com>

Co-authored-by: HParker <HParker@github.com>
Co-authored-by: Dinah Shi <dinahshi@github.com>

Author: 3 people

yjit_codegen.c

@@ -1012,7 +1012,8 @@ gen_get_ivar(jitstate_t *jit, ctx_t *ctx, const int max_chain_depth, VALUE compt
     // NOTE: This assumes nobody changes the allocator of the class after allocation.
     //       Eventually, we can encode whether an object is T_OBJECT or not
     //       inside object shapes.
-    if (rb_get_alloc_func(comptime_val_klass) != rb_class_allocate_instance) {
+    if (!RB_TYPE_P(comptime_receiver, T_OBJECT) ||
+            rb_get_alloc_func(comptime_val_klass) != rb_class_allocate_instance) {
         // General case. Call rb_ivar_get(). No need to reconstruct interpreter
         // state since the routine never raises exceptions or allocate objects
         // visibile to Ruby.