Add a version of CompiledIC_at that doesn't create a new RelocIterator

Creation of RelocIterators show up high in profiles of the remark phase, in the G1 Class Unloading project.

There's a pattern in the nmethod/codecache code to create a RelocIterator and then materialize a CompiledIC:

    RelocIterator iter(this, low_boundary);
    while(iter.next()) {
      if (iter.type() == relocInfo::virtual_call_type) {
        CompiledIC *ic = CompiledIC_at(iter.reloc());

CompiledIC_at is implemented as:
  new CompiledIC(call_site->code(), nativeCall_at(call_site->addr()));

And one of the first thing CompiledIC::CompiledIC(const nmethod* nm, NativeCall* call) does is to create a new RelocIterator:
...
address ic_call = call->instruction_address();
...
  RelocIterator iter(nm, ic_call, ic_call+1);
  bool ret = iter.next();
  assert(ret == true, "relocInfo must exist at this address");
  assert(iter.addr() == ic_call, "must find ic_call");

I would like to propose that we pass down the RelocIterator that we already have, instead of creating a new.