[Mem2Reg] Respect optnone

Mem2Reg shouldn't be optimizing a function that is marked
optnone. There is a test checking this that fails when mem2reg is
explicitly added to the standard pass pipeline.

llvm-svn: 255336
diff --git a/llvm/lib/Transforms/Utils/Mem2Reg.cpp b/llvm/lib/Transforms/Utils/Mem2Reg.cpp
index 00cf4e6..aa1e35d 100644
--- a/llvm/lib/Transforms/Utils/Mem2Reg.cpp
+++ b/llvm/lib/Transforms/Utils/Mem2Reg.cpp
@@ -63,6 +63,9 @@
 
   BasicBlock &BB = F.getEntryBlock();  // Get the entry node for the function
 
+  if (F.hasFnAttribute(Attribute::OptimizeNone))
+    return false;
+
   bool Changed  = false;
 
   DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();