[NFC] PrunedLiveness: Clarified boundary API.

When checking whether an instruction is contained in a liveness
boundary, a pointer to a DeadEndBlocks instance must always be passed.
When the pointer is null, it is only checked that the instruction occurs
within the direct live region.  When the pointer is non-null, it is
checked whether the instruction occurs within the region obtained by
extending the live region up to the availability boundary within
dead-end regions that are adjacent to the non-lifetime-ending portion of
the liveness boundary.

lib/SIL/Verifier/LoadBorrowImmutabilityChecker.cpp

@@ -337,7 +337,7 @@ bool LoadBorrowImmutabilityAnalysis::isImmutableInScope(
       continue;
     }
 
-    if (borrowLiveness.isWithinBoundary(write)) {
+    if (borrowLiveness.isWithinBoundary(write, /*deadEndBlocks=*/nullptr)) {
       llvm::errs() << "Write: " << *write;
       return false;
     }