[ARM] Fix Scavenger assert due to underestimated stack size

(this is resubmit of r268529 with minor refactoring. r268529 was reverted
 at r268536 due a memory sanitizer failure.  I have not been able to
 reproduce that failure and I checked all the variable used in my change
 but I could not spot an issue. I did some refactoring and see if it will
 give a clearer hint)

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: 268810