Rename singleton InstId constants to TypeInstId (#5323)

These constant instructions are all TypeInstId already in their type,
and this makes their names match.

Change the name of MakeSingletonInstId as well and update its comment.

toolchain/check/handle_if_expr.cpp

@@ -38,7 +38,7 @@ static auto DecayIntLiteralToSizedInt(Context& context, Parse::NodeId node_id,
                                       SemIR::InstId operand_id)
     -> SemIR::InstId {
   if (context.types().GetInstId(context.insts().Get(operand_id).type_id()) ==
-      SemIR::IntLiteralType::InstId) {
+      SemIR::IntLiteralType::TypeInstId) {
     operand_id = ConvertToValueOfType(
         context, node_id, operand_id,
         MakeIntType(context, node_id, SemIR::IntKind::Signed,