Emitting an instruction in Subzero requires a fair amount of
boilerplated code:

Context.insert(<InstType>::create(Func, <Args>...));

The ordeal is worse if one needs access to the recently create
instructionL

auto *Instr = <InstType>::create(Func, <Args>...);
Context.insert(Instr);
Instr->...

This CL introduces a new LoweringContext::insert() method:

template <<InstType>, <Args>...>
<InstType> *LoweringContext::insert(<Args>...) {
  auto *New = Inst::create(Node.Cfg, <Args>...);
  insert(New);
  return New;
}

This is essentially a syntatic sugar that allows instructions to be
emitted by using

Context.insert<InstType>(<Args>...);

The compiler should be able to inline the calls (and get rid of the
return value) when appropriate.

make bloat reviews a small increase in translator code size

BUG=
R=sehr@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/1527143003 .