[IR] Create new method in `Function` class (NFC)

Create method `optForNone()` testing for the function level equivalent of
`-O0` and refactor appropriately.

Differential revision: https://reviews.llvm.org/D59852

llvm-svn: 357638
diff --git a/llvm/lib/Transforms/Scalar/WarnMissedTransforms.cpp b/llvm/lib/Transforms/Scalar/WarnMissedTransforms.cpp
index 73687e1..bcb877a 100644
--- a/llvm/lib/Transforms/Scalar/WarnMissedTransforms.cpp
+++ b/llvm/lib/Transforms/Scalar/WarnMissedTransforms.cpp
@@ -92,7 +92,7 @@
 WarnMissedTransformationsPass::run(Function &F, FunctionAnalysisManager &AM) {
   // Do not warn about not applied transformations if optimizations are
   // disabled.
-  if (F.hasFnAttribute(Attribute::OptimizeNone))
+  if (F.optForNone())
     return PreservedAnalyses::all();
 
   auto &ORE = AM.getResult<OptimizationRemarkEmitterAnalysis>(F);