[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

@@ -1910,11 +1910,15 @@ void swift::visitTransitiveEndBorrows(
 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;
+  SmallVector<SILValue, 8> roots;
+  findGuaranteedReferenceRoots(value, 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;
+  });
 }