Subzero: Use range-based for loops with llvm::ilist<Inst> lists.

This reestablishes C++11 features lost after switching to llvm::ilist<> in two previous CLs:
https://codereview.chromium.org/709533002/
https://codereview.chromium.org/794923002/

BUG= none
R=jfb@chromium.org, jvoung@chromium.org

Review URL: https://codereview.chromium.org/819403002
5 files changed