Restructure action-dependence APIs (#7074)

- Rename `ActionIsDependent` to `ActionIsPerformable` (with negated
meaning), because that name is more concrete and, um, actionable.
- Replace `OperandIsDependent` with `OperandDependence`, which returns a
`ConstantDependence` instead of a bool. We need this additional
generality for handling form actions, where we sometimes need to ask
whether something has _any_ dependence, not just whether it has template
dependence.

toolchain/check/convert.cpp

@@ -1971,17 +1971,18 @@ auto Convert(Context& context, SemIR::LocId loc_id, SemIR::InstId expr_id,
   // didn't apply in the template definition.
   // TODO: Support this for targets other than `Value`.
   if (sem_ir.insts().Get(expr_id).type_id() != target.type_id &&
-      target.kind == ConversionTarget::Value &&
-      (OperandIsDependent(context, expr_id) ||
-       OperandIsDependent(context, target.type_id))) {
+      target.kind == ConversionTarget::Value) {
     auto target_type_inst_id = context.types().GetTypeInstId(target.type_id);
-    return AddDependentActionSplice(
-        context, loc_id,
-        SemIR::ConvertToValueAction{
-            .type_id = GetSingletonType(context, SemIR::InstType::TypeInstId),
-            .inst_id = expr_id,
-            .target_type_inst_id = target_type_inst_id},
-        target_type_inst_id);
+    SemIR::ConvertToValueAction convert_action = {
+        .type_id = GetSingletonType(context, SemIR::InstType::TypeInstId),
+        .inst_id = expr_id,
+        .target_type_inst_id = target_type_inst_id};
+    // We don't use `HandleAction` here because it would call `PerformAction`
+    // inline if it's performable, which would lead to infinite recursion.
+    if (!ActionIsPerformable(context, convert_action)) {
+      return AddDependentActionSplice(context, loc_id, convert_action,
+                                      target_type_inst_id);
+    }
   }
 
   // If this is not a builtin conversion, try an `ImplicitAs` conversion.