[OwnershipUtils] Extracted utility for common use.

Promoted isRedundantLexicalBeginBorrow function to OwnershipUtils so
that it can be used in more than just SemanticARCOpts.

Author: nate-chandler

include/swift/SIL/OwnershipUtils.h

@@ -1206,6 +1206,14 @@ void visitTransitiveEndBorrows(
     BorrowedValue beginBorrow,
     function_ref<void(EndBorrowInst *)> visitEndBorrow);
 
+/// Whether the specified lexical begin_borrow instruction is nested.
+///
+/// A begin_borrow [lexical] is nested if the borrowed value's lifetime is
+/// guaranteed by another lexical scope.  That happens if:
+/// - the value is a guaranteed argument to the function
+/// - the value is itself a begin_borrow [lexical]
+bool isNestedLexicalBeginBorrow(BeginBorrowInst *bbi);
+
 } // namespace swift
 
 #endif

lib/SIL/Utils/OwnershipUtils.cpp

@@ -1487,3 +1487,21 @@ void swift::visitTransitiveEndBorrows(
     }
   }
 }
+
+/// Whether the specified lexical begin_borrow instruction is nested.
+///
+/// A begin_borrow [lexical] is nested if the borrowed value's lifetime is
+/// guaranteed by another lexical scope.  That happens if:
+/// - the value is a guaranteed argument to the function
+/// - the value is itself a begin_borrow [lexical]
+bool swift::isNestedLexicalBeginBorrow(BeginBorrowInst *bbi) {
+  assert(bbi->isLexical());
+  auto value = bbi->getOperand();
+  if (auto *outerBBI = dyn_cast<BeginBorrowInst>(value)) {
+    return outerBBI->isLexical();
+  }
+  if (auto *arg = dyn_cast<SILFunctionArgument>(value)) {
+    return arg->getOwnershipKind() == OwnershipKind::Guaranteed;
+  }
+  return false;
+}

lib/SILOptimizer/SemanticARC/BorrowScopeOpts.cpp

@@ -24,32 +24,14 @@
 using namespace swift;
 using namespace swift::semanticarc;
 
-/// Whether the provided lexical begin_borrow instruction is redundant.
-///
-/// A begin_borrow [lexical] is redundant if the borrowed value's lifetime is
-/// otherwise guaranteed.  That happens if:
-/// - the value is a guaranteed argument to the function
-/// - the value is itself a begin_borrow [lexical]
-static bool isRedundantLexicalBeginBorrow(BeginBorrowInst *bbi) {
-  assert(bbi->isLexical());
-  auto value = bbi->getOperand();
-  if (auto *outerBBI = dyn_cast<BeginBorrowInst>(value)) {
-    return outerBBI->isLexical();
-  }
-  if (auto *arg = dyn_cast<SILFunctionArgument>(value)) {
-    return arg->getOwnershipKind() == OwnershipKind::Guaranteed;
-  }
-  return false;
-}
-
 bool SemanticARCOptVisitor::visitBeginBorrowInst(BeginBorrowInst *bbi) {
   // Quickly check if we are supposed to perform this transformation.
   if (!ctx.shouldPerform(ARCTransformKind::RedundantBorrowScopeElimPeephole))
     return false;
 
   // Non-redundant, lexical borrow scopes must remain in order to ensure that
   // value lifetimes are not observably shortened.
-  if (bbi->isLexical() && !isRedundantLexicalBeginBorrow(bbi)) {
+  if (bbi->isLexical() && !isNestedLexicalBeginBorrow(bbi)) {
     return false;
   }