[LifetimeCompletion] Avoid instruction list walk.

When computing the available region, a forward walk is done from the
non-lifetime-ending boundary of the live region.  That boundary consists
of (1) the target blocks of boundary edges, (2) dead defs, and (3) last
users which are non-consuming.  This forward walk is done at the block
level, so neither the specific dead def (for (2)) nor specific last
non-consuming user (for (3)) is required to perform the walk; indeed
currently these are computed and then immediately used only to obtain
the blocks in which they appear and then discarded.  Avoid computing the
specific dead defs and specific last non-consuming users by switching to
a lower-fidelity liveness boundary computation via
`PrunedLivenessBlockBoundary`.

Author: nate-chandler

lib/SIL/Utils/OSSALifetimeCompletion.cpp

@@ -262,9 +262,18 @@ void AvailabilityBoundaryVisitor::visit(const SSAPrunedLiveness &liveness,
 void AvailabilityBoundaryVisitor::computeRegion(
     const SSAPrunedLiveness &liveness) {
   // (1) Compute the complete liveness boundary.
-  PrunedLivenessBoundary boundary;
+  PrunedLivenessBlockBoundary boundary;
   liveness.computeBoundary(boundary);
 
+  BasicBlockSet consumingBlocks(value->getFunction());
+
+  liveness.visitUsers(
+      [&consumingBlocks](auto *instruction, auto lifetimeEnding) {
+        if (lifetimeEnding.isEnding()) {
+          consumingBlocks.insert(instruction->getParent());
+        }
+      });
+
   // Used in the forward walk below (3).
   BasicBlockWorklist regionWorklist(value->getFunction());
 
@@ -284,18 +293,14 @@ void AvailabilityBoundaryVisitor::computeRegion(
     regionWorklist.push(block);
   };
 
-  for (SILInstruction *lastUser : boundary.lastUsers) {
-    if (liveness.isInterestingUser(lastUser)
-        != PrunedLiveness::LifetimeEndingUse) {
-      collect(lastUser->getParent());
+  for (auto *endBlock : boundary.endBlocks) {
+    if (!consumingBlocks.contains(endBlock)) {
+      collect(endBlock);
     }
   }
   for (SILBasicBlock *edge : boundary.boundaryEdges) {
     collect(edge);
   }
-  for (SILNode *deadDef : boundary.deadDefs) {
-    collect(deadDef->getParentBlock());
-  }
 
   // (3) Forward walk to find the region in which `value` might be available.
   while (auto *block = regionWorklist.pop()) {