[ARM] Fix Scavenger assert due to underestimated stack size

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

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

llvm-svn: 268529