SlotIndexes used to store the index list in a crufty custom linked-list. I can't

SlotIndexes used to store the index list in a crufty custom linked-list. I can't
for the life of me remember why I wrote it this way, but I can't see any good
reason for it now. This patch replaces the custom linked list with an ilist.

This change should preserve the existing numberings exactly, so no generated code
should change (if it does, file a bug!).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@154904 91177308-0d34-0410-b5e6-96231b3b80d8