[OwnershipUtils] Match complex nested borrows.

Recognize lexical borrows as nested when their borrowee's guaranteed
reference roots are all lexical borrows.

Addresses the following regressions

Breadcrumbs.MutatedUTF16ToIdx.Mixed         188     882     +369.1%   **0.21x**
Breadcrumbs.MutatedIdxToUTF16.Mixed         230     926     +302.6%   **0.25x**

seen when enabling lexical lifetimes in the standard library.

Author: nate-chandler

lib/SIL/Utils/OwnershipUtils.cpp

@@ -2409,16 +2409,25 @@ void swift::visitTransitiveEndBorrows(
 ///
 /// 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]
+/// - the non-guaranteed borrowee's value is lexical
+/// - the guaranteed borrowee's value's reference roots are lexical
+///   - for example, the borrowee 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 (value->getOwnershipKind() != OwnershipKind::Guaranteed) {
+    return value->isLexical();
   }
-  if (auto *arg = dyn_cast<SILFunctionArgument>(value)) {
-    return arg->getOwnershipKind() == OwnershipKind::Guaranteed;
-  }
-  return false;
+  SmallVector<SILValue, 8> roots;
+  findGuaranteedReferenceRoots(value, /*lookThroughNestedBorrows=*/false,
+                               roots);
+  return llvm::all_of(roots, [](auto root) {
+    if (auto *outerBBI = dyn_cast<BeginBorrowInst>(root)) {
+      return outerBBI->isLexical();
+    }
+    if (auto *arg = dyn_cast<SILFunctionArgument>(root)) {
+      return arg->getOwnershipKind() == OwnershipKind::Guaranteed;
+    }
+    return false;
+  });
 }