2 small simplifications (#5055)

* `IsInstanceMethod` can look in the function's `self_param_id` instead
of iterating through all of the implicit parameter patterns.
* An associated entity is always associated with a single interface, so
we don't need to handle the case when it isn't.

Co-authored-by: Josh L <josh11b@users.noreply.github.com>

toolchain/check/member_access.cpp

@@ -45,15 +45,7 @@ static auto GetClassElementIndex(Context& context, SemIR::InstId element_id)
 static auto IsInstanceMethod(const SemIR::File& sem_ir,
                              SemIR::FunctionId function_id) -> bool {
   const auto& function = sem_ir.functions().Get(function_id);
-  for (auto param_id :
-       sem_ir.inst_blocks().GetOrEmpty(function.implicit_param_patterns_id)) {
-    if (SemIR::Function::GetNameFromPatternId(sem_ir, param_id) ==
-        SemIR::NameId::SelfValue) {
-      return true;
-    }
-  }
-
-  return false;
+  return function.self_param_id.has_value();
 }
 
 // Returns the highest allowed access. For example, if this returns `Protected`
@@ -187,13 +179,8 @@ static auto PerformImplLookup(
     MakeDiagnosticBuilderFn missing_impl_diagnoser = nullptr) -> SemIR::InstId {
   auto interface_type =
       GetInterfaceFromFacetType(context, assoc_type.interface_type_id);
-  if (!interface_type) {
-    context.TODO(loc_id,
-                 "Lookup of impl witness not yet supported except for a single "
-                 "interface");
-    return SemIR::ErrorInst::SingletonInstId;
-  }
-
+  // An associated entity is always associated with a single interface.
+  CARBON_CHECK(interface_type);
   auto self_type_id = context.types().GetTypeIdForTypeConstantId(type_const_id);
   auto witness_id =
       LookupImplWitness(context, loc_id, type_const_id,