[NFC] OptUtils: Promote isInLoop to internal.

It will be used by both AllocBoxToStack in addition to StackPromotion
and for the same reason.

Author: nate-chandler

SwiftCompilerSources/Sources/Optimizer/FunctionPasses/StackPromotion.swift

@@ -330,17 +330,3 @@ private extension BasicBlockRange {
     exits.contains { !deadEndBlocks.isDeadEnd($0) && !$0.hasSinglePredecessor }
   }
 }
-
-private func isInLoop(block startBlock: BasicBlock, _ context: FunctionPassContext) -> Bool {
-  var worklist = BasicBlockWorklist(context)
-  defer { worklist.deinitialize() }
-
-  worklist.pushIfNotVisited(contentsOf: startBlock.successors)
-  while let block = worklist.pop() {
-    if block == startBlock {
-      return true
-    }
-    worklist.pushIfNotVisited(contentsOf: block.successors)
-  }
-  return false
-}

SwiftCompilerSources/Sources/Optimizer/Utilities/OptUtils.swift

@@ -1012,3 +1012,17 @@ func eraseIfDead(functions: [Function], _ context: ModulePassContext) {
     toDelete = remaining
   }
 }
+
+func isInLoop(block startBlock: BasicBlock, _ context: FunctionPassContext) -> Bool {
+  var worklist = BasicBlockWorklist(context)
+  defer { worklist.deinitialize() }
+
+  worklist.pushIfNotVisited(contentsOf: startBlock.successors)
+  while let block = worklist.pop() {
+    if block == startBlock {
+      return true
+    }
+    worklist.pushIfNotVisited(contentsOf: block.successors)
+  }
+  return false
+}