Use LLVM's intrusive list ADT template to implement instruction lists.  This embeds prev/next pointers into the instruction, and as such, iterators essentially double as instruction pointers.  This means stripping off one level of indirection when dereferencing, and also the range-based for loop can't be used.

The performance difference in translation time seems to be 1-2%.

I tried to also do this for the much less used PhiList and AssignList, but ran into SFINAE problems.

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/709533002