[NFC] PrunedLiveness: Tweaked enum wrapper.

Clarify methods.  Unfortunately, without other changes I haven't
identified, the `enum class` can't be made an `enum` for ideal usage.

Author: nate-chandler

include/swift/SIL/PrunedLiveness.h

@@ -391,22 +391,20 @@ class PrunedLiveness {
       Ending,
       // The instruction doesn't use the value.
       NonUse,
-    };
-    Value value;
+    } value;
 
     LifetimeEnding(Value value) : value(value) {}
-    explicit LifetimeEnding(bool lifetimeEnding)
-        : value(lifetimeEnding ? Value::Ending : Value::NonEnding) {}
     operator Value() const { return value; }
+
+    static LifetimeEnding forUse(bool lifetimeEnding) {
+      return lifetimeEnding ? Value::Ending : Value::NonEnding;
+    }
+    bool isEnding() const { return value == Value::Ending; }
+
     LifetimeEnding meet(LifetimeEnding const other) const {
       return value < other.value ? *this : other;
     }
     void meetInPlace(LifetimeEnding const other) { *this = meet(other); }
-    bool isEnding() const { return value == Value::Ending; }
-
-    static LifetimeEnding NonUse() { return {Value::NonUse}; };
-    static LifetimeEnding Ending() { return {Value::Ending}; };
-    static LifetimeEnding NonEnding() { return {Value::NonEnding}; };
   };
 
 protected:
@@ -467,8 +465,8 @@ class PrunedLiveness {
     auto useIter = users.find(user);
     if (useIter == users.end())
       return NonUser;
-    return useIter->second == LifetimeEnding::Ending() ? LifetimeEndingUse
-                                                       : NonLifetimeEndingUse;
+    return useIter->second.isEnding() ? LifetimeEndingUse
+                                      : NonLifetimeEndingUse;
   }
 
   using ConstUserRange =

lib/SIL/Utils/PrunedLiveness.cpp

@@ -216,12 +216,12 @@ void PrunedLiveRange<LivenessWithDefs>::updateForUse(
 template <typename LivenessWithDefs>
 void PrunedLiveRange<LivenessWithDefs>::updateForUse(SILInstruction *user,
                                                      bool lifetimeEnding) {
-  updateForUse(user, LifetimeEnding(lifetimeEnding));
+  updateForUse(user, LifetimeEnding::forUse(lifetimeEnding));
 }
 
 template <typename LivenessWithDefs>
 void PrunedLiveRange<LivenessWithDefs>::extendToNonUse(SILInstruction *inst) {
-  updateForUse(inst, LifetimeEnding::NonUse());
+  updateForUse(inst, LifetimeEnding::Value::NonUse);
 }
 
 template <typename LivenessWithDefs>