Add utilities to support OSSA update after running SSAUpdater.

This directly adds support to BasicBlockCloner for updating OSSA.

It also adds a much more general-purpose GuaranteedPhiBorrowFixup
utility which can be used for more complicated SSA updates, in which
multiple phis need to be created. More generally, it handles adding
nested borrow scopes for guaranteed phis even when that phi is used by
other guaranteed phis.

include/swift/SILOptimizer/Utils/BasicBlockOptUtils.h

@@ -83,6 +83,19 @@ bool rotateLoop(SILLoop *loop, DominanceInfo *domInfo, SILLoopInfo *loopInfo,
                 bool rotateSingleBlockLoops, SILBasicBlock *upToBB,
                 bool shouldVerify);
 
+//===----------------------------------------------------------------------===//
+//                             BasicBlock Cloning
+//===----------------------------------------------------------------------===//
+
+/// Return true if the \p termInst can be cloned. If termInst produces a
+/// guaranteed result, then it must be possible to create a nested borrow scope
+/// for that result when the cloner generates a guaranteed phi.
+///
+/// Note: if all the terminators uses will also be cloned, then a guaranteed phi
+/// won't be necessary. This is one of the reasons that cloning a region is much
+/// better than cloning a single block at a time.
+bool canCloneTerminator(TermInst *termInst);
+
 /// Sink address projections to their out-of-block uses. This is
 /// required after cloning a block and before calling
 /// updateSSAAfterCloning to avoid address-type phis.
@@ -173,6 +186,7 @@ public:
       if (!canCloneInstruction(&inst))
         return false;
     }
+    canCloneTerminator(origBB->getTerminator());
     return true;
   }
 
@@ -221,6 +235,19 @@ public:
     bi->eraseFromParent();
   }
 
+  /// Create phis and maintain OSSA invariants.
+  ///
+  /// Note: This must be called after calling cloneBlock or cloneBranchTarget,
+  /// before using any OSSA utilities.
+  ///
+  /// The client may perform arbitrary branch fixups and dead block removal
+  /// after cloning and before calling this.
+  ///
+  /// WARNING: If client converts terminator results to phis (e.g. replaces a
+  /// switch_enum with a branch), then it must call this before performing that
+  /// transformation, or fix the OSSA representation of that value itself.
+  void updateOSSAAfterCloning();
+
   /// Get the newly cloned block corresponding to `origBB`.
   SILBasicBlock *getNewBB() {
     return remapBasicBlock(origBB);
@@ -228,10 +255,14 @@ public:
 
   bool wasCloned() { return isBlockCloned(origBB); }
 
-  /// Helper function to perform SSA updates after calling cloneBranchTarget.
-  void updateSSAAfterCloning();
-
 protected:
+  /// Helper function to perform SSA updates used by updateOSSAAfterCloning.
+  void updateSSAAfterCloning(SmallVectorImpl<SILPhiArgument *> &newPhis);
+
+  /// Given a terminator result, either from the original or the cloned block,
+  /// update OSSA for any phis created for the result during edge splitting.
+  void updateOSSATerminatorResult(SILPhiArgument *termResult);
+
   // MARK: CRTP overrides.
 
   /// Override getMappedValue to allow values defined outside the block to be