[move-function] When processing object lets only look through non-lexical borrows.

A lexical borrow is creating a new entity that should be checked separately from
the operand of the lexical borrow.

Author: gottesmm

lib/SILOptimizer/Mandatory/MoveKillsCopyableValuesChecker.cpp

@@ -127,8 +127,16 @@ bool CheckerLivenessInfo::compute() {
         consumingUse.insert(use);
         break;
       case OperandOwnership::Borrow: {
-        bool failed = !liveness.updateForBorrowingOperand(use);
-        assert(!failed && "Shouldn't see reborrows this early in the pipeline");
+        if (auto *bbi = dyn_cast<BeginBorrowInst>(user)) {
+          // Only add borrows to liveness if the borrow isn't lexical. If it is
+          // a lexical borrow, we have created an entirely new source level
+          // binding that should be tracked separately.
+          if (!bbi->isLexical()) {
+            bool failed = !liveness.updateForBorrowingOperand(use);
+            assert(!failed &&
+                   "Shouldn't see reborrows this early in the pipeline");
+          }
+        }
         break;
       }
       case OperandOwnership::ForwardingBorrow:

test/SILOptimizer/move_function_kills_copyable_addressonly_lets.swift

@@ -277,3 +277,10 @@ public func performMoveOnOneEltOfTPair2<T>(_ p: __owned TPair<T>) {
     let _ = _move(p.x) // expected-error {{_move applied to value that the compiler does not support checking}}
     nonConsumingUse(p.y)
 }
+
+public func multipleVarsWithSubsequentBorrows<T : Equatable>(_ p: T) -> Bool {
+    let k = p
+    let k2 = k
+    let k3 = _move(k)
+    return k2 == k3
+}

test/SILOptimizer/move_function_kills_copyable_values.swift

@@ -248,3 +248,10 @@ public func performMoveOnVarGlobalError() {
 public func performMoveOnLetGlobalError() {
     let _ = _move(myVarGlobal) // expected-error {{_move applied to value that the compiler does not support checking}}
 }
+
+public func multipleVarsWithSubsequentBorrows() -> Bool {
+    let k = Klass()
+    let k2 = k
+    let k3 = _move(k)
+    return k2 === k3
+}