Commit an patch from Gabor Greif in Mar 2005. This eliminates the tail

Commit an patch from Gabor Greif in Mar 2005.  This eliminates the tail
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