Remove OperandOwnership::NestedBorrow.

Now that OperandOwnership determines the operand constraints, it
doesn't make sense to distinguish between Borrow and NestedBorrow at
this level. We want these uses to automatically convert between the
nested/non-nested state as the operand's ownership changes. The use
does not need to impose any constraint on the ownership of the
incoming value.

For algorithms that need to distinguish nested borrows, it's still
trivial to do so.

Author: atrick

include/swift/SIL/SILValue.h

@@ -675,11 +675,13 @@ struct OperandOwnership {
     /// (ref_to_unowned, unchecked_trivial_bitcast)
     BitwiseEscape,
 
-    /// MARK: Uses of Owned values:
-
-    /// Borrow. Propagates the owned value within a scope, without consuming it.
+    /// Borrow. Propagates the owned or guaranteed value within a scope, without
+    /// ending its lifetime.
     /// (begin_borrow, begin_apply with @guaranteed argument)
     Borrow,
+
+    /// MARK: Uses of Owned values:
+
     /// Destroying Consume. Destroys the owned value immediately.
     /// (store, destroy, @owned destructure).
     DestroyingConsume,
@@ -689,11 +691,6 @@ struct OperandOwnership {
 
     /// MARK: Uses of Guaranteed values:
 
-    /// Nested Borrow. Propagates the guaranteed value within a nested borrow
-    /// scope, without ending the outer borrow scope, following stack
-    /// discipline.
-    /// (begin_borrow, begin_apply with @guaranteed).
-    NestedBorrow,
     /// Interior Pointer. Propagates a trivial value (e.g. address, pointer, or
     /// no-escape closure) that depends on the guaranteed value within the
     /// base's borrow scope. The verifier checks that all uses of the trivial
@@ -750,13 +747,11 @@ inline OwnershipConstraint OperandOwnership::getOwnershipConstraint() {
   case OperandOwnership::ForwardingUnowned:
   case OperandOwnership::PointerEscape:
   case OperandOwnership::BitwiseEscape:
-    return {OwnershipKind::Any, UseLifetimeConstraint::NonLifetimeEnding};
   case OperandOwnership::Borrow:
-    return {OwnershipKind::Owned, UseLifetimeConstraint::NonLifetimeEnding};
+    return {OwnershipKind::Any, UseLifetimeConstraint::NonLifetimeEnding};
   case OperandOwnership::DestroyingConsume:
   case OperandOwnership::ForwardingConsume:
     return {OwnershipKind::Owned, UseLifetimeConstraint::LifetimeEnding};
-  case OperandOwnership::NestedBorrow:
   case OperandOwnership::InteriorPointer:
   case OperandOwnership::ForwardingBorrow:
     return {OwnershipKind::Guaranteed,

lib/SIL/IR/SILValue.cpp

@@ -333,7 +333,6 @@ static bool canAcceptUnownedValue(OperandOwnership operandOwnership) {
   case OperandOwnership::Borrow:
   case OperandOwnership::DestroyingConsume:
   case OperandOwnership::ForwardingConsume:
-  case OperandOwnership::NestedBorrow:
   case OperandOwnership::InteriorPointer:
   case OperandOwnership::ForwardingBorrow:
   case OperandOwnership::EndBorrow:
@@ -412,8 +411,6 @@ StringRef OperandOwnership::asString() const {
     return "destroying-consume";
   case OperandOwnership::ForwardingConsume:
     return "forwarding-consume";
-  case OperandOwnership::NestedBorrow:
-    return "nested-borrow";
   case OperandOwnership::InteriorPointer:
     return "interior-pointer";
   case OperandOwnership::ForwardingBorrow:

lib/SIL/Utils/OwnershipUtils.cpp

@@ -890,7 +890,6 @@ Optional<ForwardingOperand> ForwardingOperand::get(Operand *use) {
   case OperandOwnership::BitwiseEscape:
   case OperandOwnership::Borrow:
   case OperandOwnership::DestroyingConsume:
-  case OperandOwnership::NestedBorrow:
   case OperandOwnership::InteriorPointer:
   case OperandOwnership::EndBorrow:
   case OperandOwnership::Reborrow: