[mono][jit] Optimize calls to Type:get_IsValueType () on gshared constrained types. (#61514)

These are used for example in Span<T>:.ctor ().

Author: vargaz

src/mono/mono/mini/intrinsics.c

@@ -1894,6 +1894,33 @@ mini_emit_inst_for_method (MonoCompile *cfg, MonoMethod *cmethod, MonoMethodSign
 			MONO_ADD_INS (cfg->cbb, ins);
 		}
 		return ins;
+	} else if (cmethod->klass == mono_defaults.systemtype_class && !strcmp (cmethod->name, "get_IsValueType") &&
+			   args [0]->klass == mono_defaults.runtimetype_class) {
+		MonoClass *k1 = get_class_from_ldtoken_ins (args [0]);
+		if (k1) {
+			MonoType *t1 = m_class_get_byval_arg (k1);
+			MonoType *constraint1 = NULL;
+
+			/* Common case in gshared BCL code: t1 is a gshared type like T_INT */
+			if (mono_class_is_gparam (k1)) {
+				MonoGenericParam *gparam = t1->data.generic_param;
+				constraint1 = gparam->gshared_constraint;
+				if (constraint1) {
+					if (constraint1->type == MONO_TYPE_OBJECT) {
+						if (cfg->verbose_level > 2)
+							printf ("-> false\n");
+						EMIT_NEW_ICONST (cfg, ins, 0);
+						return ins;
+					} else if (MONO_TYPE_IS_PRIMITIVE (constraint1)) {
+						if (cfg->verbose_level > 2)
+							printf ("-> true\n");
+						EMIT_NEW_ICONST (cfg, ins, 1);
+						return ins;
+					}
+				}
+			}
+		}
+		return NULL;
 	} else if (((!strcmp (cmethod_klass_image->assembly->aname.name, "MonoMac") ||
 	            !strcmp (cmethod_klass_image->assembly->aname.name, "monotouch")) &&
 				!strcmp (cmethod_klass_name_space, "XamCore.ObjCRuntime") &&