Subzero: Fix the LLVM trunk build.

The original Ice::Inst needed a bit of template specialization so that llvm::ilist<Ice::Inst> methods wouldn't try to invoke private/deleted ctor/dtor methods.  The Subzero code copied a pattern fairly widely used in LLVM.

Recently, LLVM improved ilist<> so that this specialization is no longer needed.

BUG= none
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/2287363002 .