[NFC] PrunedLiveness: Permit postDomBlocks dupes.

According to the documentation,

```
It's ok if postDomBlocks has duplicates or extraneous blocks, as long
as they jointly post-dominate all live blocks that aren't on dead-end
paths.
```

Previously, though, duplicates resulted in trapping: each element of
`postDomBlocks` is pushed into a `BasicBlockWorklist`; when the second
occurrence of an element in `postDomBlocks` was encountered,
`BasicBlockWorklist::push` would trigger an assertion failure.

Here, `pushIfNotVisited` is called instead.

Author: nate-chandler

lib/SIL/Utils/PrunedLiveness.cpp

@@ -860,7 +860,7 @@ void PrunedLiveRange<LivenessWithDefs>::computeBoundary(
   // Visit each post-dominating block as the starting point for a
   // backward CFG traversal.
   for (auto *block : postDomBlocks) {
-    blockWorklist.push(block);
+    blockWorklist.pushIfNotVisited(block);
   }
   while (auto *block = blockWorklist.pop()) {
     // Process each block that has not been visited and is not LiveOut.