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/eval.cpp

@@ -545,7 +545,8 @@ static auto GetConstantValue(EvalContext& eval_context,
   }
 
   // Otherwise, this is a normal instruction.
-  if (OperandIsDependent(eval_context.context(), inst_id)) {
+  if (OperandDependence(eval_context.context(), inst_id) ==
+      SemIR::ConstantDependence::Template) {
     *phase = LatestPhase(*phase, Phase::TemplateSymbolic);
   }
   return inst_id;