CodeGen: Canonicalize access to function attributes, NFC

Canonicalize access to function attributes to use the simpler API.

getAttributes().getAttribute(AttributeSet::FunctionIndex, Kind)
  => getFnAttribute(Kind)

getAttributes().hasAttribute(AttributeSet::FunctionIndex, Kind)
  => hasFnAttribute(Kind)

Also, add `Function::getFnStackAlignment()`, and canonicalize:

getAttributes().getStackAlignment(AttributeSet::FunctionIndex)
  => getFnStackAlignment()

llvm-svn: 229208
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index 014df6c..51bb7ab 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -4591,8 +4591,7 @@
       return DAG.getConstantFP(1.0, LHS.getValueType());
 
     const Function *F = DAG.getMachineFunction().getFunction();
-    if (!F->getAttributes().hasAttribute(AttributeSet::FunctionIndex,
-                                         Attribute::OptimizeForSize) ||
+    if (!F->hasFnAttribute(Attribute::OptimizeForSize) ||
         // If optimizing for size, don't insert too many multiplies.  This
         // inserts up to 5 multiplies.
         countPopulation(Val) + Log2_32(Val) < 7) {