Adjust a few Invalid comparisons to use is_valid (#3583)

Deliberately not touching the case that came up from #3575

toolchain/check/context.cpp

@@ -210,7 +210,7 @@ auto Context::ResolveIfLazyImportRef(SemIR::InstId inst_id) -> void {
 auto Context::LookupNameInDecl(Parse::NodeId /*parse_node*/,
                                SemIR::NameId name_id,
                                SemIR::NameScopeId scope_id) -> SemIR::InstId {
-  if (scope_id == SemIR::NameScopeId::Invalid) {
+  if (!scope_id.is_valid()) {
     // Look for a name in the current scope only. There are two cases where the
     // name would be in an outer scope:
     //

toolchain/check/decl_name_stack.cpp

@@ -66,7 +66,7 @@ auto DeclNameStack::LookupOrAddName(NameContext name_context,
       return name_context.resolved_inst_id;
 
     case NameContext::State::Unresolved:
-      if (name_context.target_scope_id == SemIR::NameScopeId::Invalid) {
+      if (!name_context.target_scope_id.is_valid()) {
         context_->AddNameToLookup(name_context.parse_node,
                                   name_context.unresolved_name_id, target_id);
       } else {

toolchain/sem_ir/formatter.cpp

@@ -338,7 +338,7 @@ class InstNamer {
       return;
     }
 
-    if (parse_node == Parse::NodeId::Invalid) {
+    if (!parse_node.is_valid()) {
       if (const auto& block = sem_ir_.inst_blocks().Get(block_id);
           !block.empty()) {
         parse_node = sem_ir_.insts().Get(block.front()).parse_node();

toolchain/sem_ir/value_stores.h

@@ -77,7 +77,7 @@ class TypeStore : public ValueStore<TypeId> {
       return InstId::BuiltinTypeType;
     } else if (type_id == TypeId::Error) {
       return InstId::BuiltinError;
-    } else if (type_id == TypeId::Invalid) {
+    } else if (!type_id.is_valid()) {
       return InstId::Invalid;
     } else {
       return Get(type_id).inst_id;