Subzero: Fix a build error against LLVM trunk.

LLVM commit 1d79fff9e65e77f84bf80c2cf4f0155bd167c90d changed the way ilist::reverse_iterator is converted to ilist::iterator.

See https://github.com/llvm-mirror/llvm/commit/1d79fff9e65e77f84bf80c2cf4f0155bd167c90d

BUG= none
R=kschimpf@google.com

Review URL: https://codereview.chromium.org/2297363002 .