[ARM] Fix Scavenger assert due to underestimated stack size

(re-apply r268810 as it exposed an uninitialized variable in ARM MFI.
 Patch 268868 should fix that.)

Summary:
Currently, when checking if a stack is "BigStack" or not, it doesn't count into spills and arguments. Therefore, LLVM won't reserve spill slot for this actually "BigStack". This may cause scavenger failure.

Reviewers: rengolin

Subscribers: vitalybuka, aemerson, rengolin, tberghammer, danalbert, srhines, llvm-commits

Differential Revision: http://reviews.llvm.org/D19896

llvm-svn: 268869