Add PrunedLiveness::computeSSALiveness

Straighforward API to build pruned liveness from a single SSA value.

This 3-line function allows PrunedLiveness to be used anywhere
ValueLifetimeAnalysis was used in OSSA form. Aside from simplicity and
efficiency, the difference is that this composes with other utilities
that only care about PrunedLiveBlocks, PrunedLiveness, or
PrunesLivenessBounary independent of how those data structures were generated.

Author: atrick

include/swift/SIL/PrunedLiveness.h

@@ -284,6 +284,9 @@ class PrunedLiveness {
 
   bool areUsesWithinBoundary(ArrayRef<Operand *> uses,
                              DeadEndBlocks &deadEndBlocks) const;
+
+  /// Compute liveness for a single SSA definition.
+  void computeSSALiveness(SILValue def);
 };
 
 /// Record the last use points and CFG edges that form the boundary of

lib/SIL/Utils/PrunedLiveness.cpp

@@ -161,6 +161,17 @@ bool PrunedLiveness::areUsesWithinBoundary(ArrayRef<Operand *> uses,
   return true;
 }
 
+// An SSA def meets all the criteria for pruned liveness--def dominates all uses
+// with no holes in the liverange. The lifetime-ending uses are also
+// recorded--destroy_value or end_borrow. However destroy_values may not
+// jointly-post dominate if dead-end blocks are present.
+void PrunedLiveness::computeSSALiveness(SILValue def) {
+  initializeDefBlock(def->getParentBlock());
+  for (Operand *use : def->getUses()) {
+    updateForUse(use->getUser(), use->isLifetimeEnding());
+  }
+}
+
 void PrunedLivenessBoundary::visitInsertionPoints(
     llvm::function_ref<void(SILBasicBlock::iterator insertPt)> visitor,
     DeadEndBlocks *deBlocks) {