[Mem2Reg] NFC: Lifted livePhiBlocks into caller.

Previously, the value was computed and destroyed during
`promoteAllocationInBlock`.  Make the value available in
`StackAllocationPromoter`'s top-level routine (`::run`) in preparation
for using it there.

lib/SILOptimizer/Transforms/SILMem2Reg.cpp

@@ -938,7 +938,7 @@ public:
 
 private:
   /// Promote AllocStacks into SSA.
-  void promoteAllocationToPhi();
+  void promoteAllocationToPhi(BlockSetVector &livePhiBlocks);
 
   /// Replace the dummy nodes with new block arguments.
   void addBlockArguments(BlockSetVector &phiBlocks);
@@ -1685,7 +1685,8 @@ void StackAllocationPromoter::pruneAllocStackUsage() {
   LLVM_DEBUG(llvm::dbgs() << "*** Finished pruning : " << *asi);
 }
 
-void StackAllocationPromoter::promoteAllocationToPhi() {
+void StackAllocationPromoter::promoteAllocationToPhi(
+    BlockSetVector &livePhiBlocks) {
   LLVM_DEBUG(llvm::dbgs() << "*** Placing Phis for : " << *asi);
 
   // A list of blocks that will require new Phi values.
@@ -1781,10 +1782,6 @@ void StackAllocationPromoter::promoteAllocationToPhi() {
   // Replace the dummy values with new block arguments.
   addBlockArguments(phiBlocks);
 
-  // The blocks which still have new phis after fixBranchesAndUses runs.  These
-  // are not necessarily the same as phiBlocks because fixBranchesAndUses
-  // removes superfluous proactive phis.
-  BlockSetVector livePhiBlocks(asi->getFunction());
   // Hook up the Phi nodes, loads, and debug_value_addr with incoming values.
   fixBranchesAndUses(phiBlocks, livePhiBlocks);
 
@@ -1801,8 +1798,13 @@ void StackAllocationPromoter::run() {
   // per block and the last store is recorded.
   pruneAllocStackUsage();
 
+  // The blocks which still have new phis after fixBranchesAndUses runs.  These
+  // are not necessarily the same as phiBlocks because fixBranchesAndUses
+  // removes superfluous proactive phis.
+  BlockSetVector livePhiBlocks(asi->getFunction());
+
   // Replace AllocStacks with Phi-nodes.
-  promoteAllocationToPhi();
+  promoteAllocationToPhi(livePhiBlocks);
 
   // Make sure that all of the allocations were promoted into registers.
   assert(isWriteOnlyAllocation(asi) && "Non-write uses left behind");