Move the segmented stack switch to a function attribute

This removes the -segmented-stacks command line flag in favor of a
per-function "split-stack" attribute.

Patch by Luqman Aden and Alex Crichton!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205997 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/MachineFunction.h

@@ -259,6 +259,9 @@ public:
     return MBBNumbering[N];
   }
 
+  /// Should we be emitting segmented stack stuff for the function
+  bool shouldSplitStack();
+
   /// getNumBlockIDs - Return the number of MBB ID's allocated.
   ///
   unsigned getNumBlockIDs() const { return (unsigned)MBBNumbering.size(); }