Migrate to OwnershipLifetimeExtender API: borrowCopyOverScope etc.

Preparation for rewriting non-trivial terminators and generalizing
support for guaranteed phis.

Add guaranteedUsePoints to the RAUW context. This will replace ALL
existing context book-keeping once the old code is deleted.

Introduce a borrowCopyOverScope entry point to handle extending
lifetime over a BorrowedValue. This simply uses the
BorrowedLifetimeExtender.

Introduce higher-level APIs:
- borrowOverValue to extened over a guaranteedValue
- borrowOverSingleUse to extened over a single guaranteed use

These replace both createPlusZeroBorrow and createPlusOneBorrow.

Update RAUW-ctor, RAUW::handleUnowned, and replaceAddressUses to use
the new API.

Restructure RAUW::canFixUpOwnershipForRAUW. Simply use
findInnerTransitiveGuaranteedUses.

Replace RAUW::handleGuaranteed and rewriteReborrows with
OLE::borrowOverValue.

Use the BorrowedLifetimeExtender utility to handle all situations
correctly.

TODO: createPlusOneBorrow can be completely removed, and a massive
amount of confusing/incomplete code can be deleted in a follow-up
commit.

Author: atrick

include/swift/SILOptimizer/Utils/OwnershipOptUtils.h

@@ -148,6 +148,10 @@ struct OwnershipFixupContext {
   InstModCallbacks &callbacks;
   DeadEndBlocks &deBlocks;
 
+  // Cache the use-points for the lifetime of an inner guaranteed value (which
+  // does not introduce a borrow scope) after checking validity. These will be
+  // used again to extend the lifetime of the replacement value.
+  SmallVector<Operand *, 8> guaranteedUsePoints;
 
   // FIXME: remove these two vectors once BorrowedLifetimeExtender is used
   // everywhere.
@@ -162,6 +166,8 @@ struct OwnershipFixupContext {
     /// compute all transitive address uses of oldValue. If we find that we do
     /// need this fixed up, then we will copy our interior pointer base value
     /// and use this to seed that new lifetime.
+    ///
+    /// FIXME: shouldn't these already be covered by guaranteedUsePoints?
     SmallVector<Operand *, 8> allAddressUsesFromOldValue;
 
     /// This is the interior pointer (e.g. ref_element_addr)
@@ -184,6 +190,7 @@ struct OwnershipFixupContext {
       : callbacks(callbacks), deBlocks(deBlocks) {}
 
   void clear() {
+    guaranteedUsePoints.clear();
     transitiveBorrowedUses.clear();
     recursiveReborrows.clear();
     extraAddressFixupInfo.allAddressUsesFromOldValue.clear();