[SILOpt] Fix build by only accessing seenUse in non-assert builds

Author: ahoppen

include/swift/SILOptimizer/Utils/PrunedLiveness.h

@@ -145,7 +145,7 @@ class PrunedLiveBlocks {
 public:
   bool empty() const { return liveBlocks.empty(); }
 
-  void clear() { liveBlocks.clear(); seenUse = false; }
+  void clear() { liveBlocks.clear(); SWIFT_ASSERT_ONLY(seenUse = false); }
 
   void initializeDefBlock(SILBasicBlock *defBB) {
     assert(!seenUse && "cannot initialize more defs with partial liveness");

lib/SILOptimizer/Utils/PrunedLiveness.cpp

@@ -50,7 +50,7 @@ void PrunedLiveBlocks::computeUseBlockLiveness(SILBasicBlock *userBB) {
 ///
 /// Terminators are not live out of the block.
 PrunedLiveBlocks::IsLive PrunedLiveBlocks::updateForUse(Operand *use) {
-  seenUse = true;
+  SWIFT_ASSERT_ONLY(seenUse = true);
 
   auto *bb = use->getUser()->getParent();
   switch (getBlockLiveness(bb)) {