pointer from ilist, storing it in the prev pointer of the first node in the
list instead.
This shrinks ilist from 8 to 4 bytes, BasicBlock from 40->36 bytes, Function
from 76->68 bytes, Module from 52->44 bytes.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@36210 91177308-0d34-0410-b5e6-96231b3b80d8