[Mem2Reg] Skipped unreachable blocks.

Uses of an alloc_stack instruction in an unreachable block must also be
unreachable.  Don't waste time optimizing unreachable code.

Author: nate-chandler

lib/SILOptimizer/Transforms/SILMem2Reg.cpp

@@ -1733,6 +1733,10 @@ bool MemoryToRegisters::run() {
     f.verifyCriticalEdges();
 
   for (auto &block : f) {
+    // Don't waste time optimizing unreachable blocks.
+    if (!domInfo->isReachableFromEntry(&block)) {
+      continue;
+    }
     for (SILInstruction *inst : deleter.updatingReverseRange(&block)) {
       auto *asi = dyn_cast<AllocStackInst>(inst);
       if (!asi)