Use more compact storage for impl lookup buckets.

If the bucket is of size zero or one, which is expected to be the common
case, then store it directly. For the remaining cases, use a side table
of lookup buckets.

toolchain/check/generic.cpp

@@ -437,11 +437,7 @@ auto RequireGenericParams(Context& context, SemIR::InstBlockId block_id)
       // constructing a generic based on it. Note this is updating the param
       // refs block, not the actual params block, so will not be directly
       // reflected in SemIR output.
-      inst_id = context.AddInstInNoBlock<SemIR::Param>(
-          context.insts().GetLocId(inst_id),
-          {.type_id = SemIR::TypeId::Error,
-           .name_id = SemIR::NameId::Base,
-           .runtime_index = SemIR::RuntimeParamIndex::Invalid});
+      inst_id = SemIR::InstId::BuiltinError;
     }
   }
 }

toolchain/check/handle_impl.cpp

@@ -278,9 +278,9 @@ static auto BuildImplDecl(Context& context, Parse::AnyImplDeclId node_id,
       {.self_id = self_type_id, .constraint_id = constraint_type_id}};
 
   // Add the impl declaration.
-  auto& lookup_bucket = context.impls().GetOrAddLookupBucket(
+  auto lookup_bucket_ref = context.impls().GetOrAddLookupBucket(
       impl_info.self_id, impl_info.constraint_id);
-  for (auto prev_impl_id : lookup_bucket) {
+  for (auto prev_impl_id : lookup_bucket_ref) {
     if (MergeImplRedecl(context, impl_info, prev_impl_id)) {
       impl_decl.impl_id = prev_impl_id;
       break;
@@ -291,7 +291,7 @@ static auto BuildImplDecl(Context& context, Parse::AnyImplDeclId node_id,
   if (!impl_decl.impl_id.is_valid()) {
     impl_info.generic_id = FinishGenericDecl(context, impl_decl_id);
     impl_decl.impl_id = context.impls().Add(impl_info);
-    lookup_bucket.push_back(impl_decl.impl_id);
+    lookup_bucket_ref.push_back(impl_decl.impl_id);
   } else {
     FinishGenericRedecl(context, impl_decl_id,
                         context.impls().Get(impl_decl.impl_id).generic_id);