[mono] Handle enum return type when inlining CreateInstance (#91061)

Use underlying base type when deciding how to inline a
CreateInstance invocation in mini_emit_inst_for_method.

Fixes #90292
(Mono abort causing .NET 8 msbuild regression).

Co-authored-by: Ulrich Weigand <[email protected]>
Co-authored-by: Larry Ewing <[email protected]>

Author: 3 people

src/mono/mono/mini/intrinsics.c

@@ -2079,7 +2079,9 @@ mini_emit_inst_for_method (MonoCompile *cfg, MonoMethod *cmethod, MonoMethodSign
 			MonoType *t = method_context->method_inst->type_argv [0];
 			MonoClass *arg0 = mono_class_from_mono_type_internal (t);
 			if (m_class_is_valuetype (arg0) && !mono_class_has_default_constructor (arg0, FALSE)) {
-				if (m_class_is_primitive (arg0)) {
+				if (m_class_is_primitive (arg0) || m_class_is_enumtype (arg0)) {
+					if (m_class_is_enumtype (arg0))
+						t = mono_class_enum_basetype_internal (arg0);
 					int dreg = alloc_dreg (cfg, mini_type_to_stack_type (cfg, t));
 					mini_emit_init_rvar (cfg, dreg, t);
 					ins = cfg->cbb->last_ins;