Force fixed-size but large alloca objects to the dynamically allocated

Force fixed-size but large alloca objects to the dynamically allocated
area to avoid using up precious stack space within the 4095 offset limit
from %fp.  Such objects that would themselves live at a large offset
were being put there already so this is a simple change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@7066 91177308-0d34-0410-b5e6-96231b3b80d8