[NFC] Rename a variable for clarity.

Author: atrick

SwiftCompilerSources/Sources/Optimizer/FunctionPasses/LifetimeDependenceScopeFixup.swift

@@ -838,12 +838,13 @@ extension ExtendableScope {
     // Collect the original end instructions and extend the range to to cover them. The resulting access scope
     // must cover the original scope because it may protect other memory operations.
     let endsToErase = self.endInstructions
-    var unusedEnds = InstructionSet(context)
+    // Track range ending instructions that have not been reused for later deletion.
+    var unreusedEnds = InstructionSet(context)
     for end in endsToErase {
       assert(range.inclusiveRangeContains(end))
-      unusedEnds.insert(end)
+      unreusedEnds.insert(end)
     }
-    defer { unusedEnds.deinitialize() }
+    defer { unreusedEnds.deinitialize() }
     for end in range.ends {
       let location = end.location.asAutoGenerated
       switch end {
@@ -861,9 +862,9 @@ extension ExtendableScope {
       default:
         break
       }
-      if unusedEnds.contains(end) {
-        unusedEnds.erase(end)
-        assert(!unusedEnds.contains(end))
+      if unreusedEnds.contains(end) {
+        unreusedEnds.erase(end)
+        assert(!unreusedEnds.contains(end))
         continue
       }
       Builder.insert(after: end, location: location, context) {
@@ -875,7 +876,7 @@ extension ExtendableScope {
       let builder = Builder(before: exitInst, location: location, context)
       range.insert(createEndInstruction(builder, context))
     }
-    return endsToErase.filter { unusedEnds.contains($0) }
+    return endsToErase.filter { unreusedEnds.contains($0) }
   }
 
   /// Create a scope-ending instruction at 'builder's insertion point.