This change allows select to have no cases.

Prior to this change, both the verifier and the select constructors expect such IR node to have at least one case.
However, bdd_simplification_pass can generate select nodes with no cases (line 430).

Since a select node with no cases (but with a default one) has a well defined semantics (i.e., always propagate the default case), we opted to allow it in the XLS IR.

PiperOrigin-RevId: 698845577

xls/ir/nodes.cc

@@ -946,7 +946,9 @@ Select::Select(const SourceInfo& loc, Node* selector,
                absl::Span<Node* const> cases,
                std::optional<Node*> default_value, std::string_view name,
                FunctionBase* function)
-    : Node(Op::kSel, cases[0]->GetType(), loc, name, function),
+    : Node(Op::kSel,
+           cases.empty() ? (*default_value)->GetType() : cases[0]->GetType(),
+           loc, name, function),
       cases_size_(cases.size()),
       has_default_value_(default_value.has_value()) {
   CHECK(IsOpClass<Select>(op_))

xls/ir/verify_node.cc

@@ -953,7 +953,9 @@ class NodeChecker : public DfsVisitor {
     XLS_RETURN_IF_ERROR(ExpectHasBitsType(sel->selector()));
     const int64_t selector_width = sel->selector()->BitCountOrDie();
     const int64_t minimum_selector_width =
-        Bits::MinBitCountUnsigned(sel->cases().size() - 1);
+        sel->cases().empty()
+            ? 0
+            : Bits::MinBitCountUnsigned(sel->cases().size() - 1);
     const bool power_of_2_cases = IsPowerOfTwo(sel->cases().size());
     if (selector_width < minimum_selector_width) {
       return absl::InternalError(absl::StrFormat(