[IRGen] Pack: Fix phi source block.

Rather than the block that we _think_ we're emitting into, use the block
we're actually emitting into.  These aren't the same because callees
can and do introduce control flow.

Author: nate-chandler

lib/IRGen/GenPack.cpp

@@ -430,7 +430,7 @@ static void emitPackExpansionPack(
   auto *next = IGF.Builder.CreateAdd(phi,
                                      llvm::ConstantInt::get(IGF.IGM.SizeTy, 1));
 
-  phi->addIncoming(next, loop);
+  phi->addIncoming(next, IGF.Builder.GetInsertBlock());
 
   // Repeat the loop.
   IGF.Builder.CreateBr(check);