This CL inlines the following builtins into TurboFan

  - %MapIteratorPrototype%.next
  - %SetIteratorPrototype%.next

following the design that we are using for Array iteration already
(different instance types for the different kinds of iterators). Details
can be found in the relevant design document at:

  https://docs.google.com/document/d/13z1fvRVpe_oEroplXEEX0a3WK94fhXorHjcOMsDmR-8

The key to great performance here is to ensure that the inlined code
allows escape analysis and scalar replacement of aggregates to remove
the allocations for the iterator itself as well as the iterator results
and potential key/value arrays in the simple case of a for-of loop (and
by extension also in other constructs that reduce to for-of loops
internally), i.e.:

  const s = new Set;
  // ... do something with s
  for (const x of s) {
    // ...
  }

Here the for-of loop shouldn't perform any allocations of helper
objects.

Drive-by-fix: Replace the ExistsJSMapWithness in JSBuiltinReducer with a more
general HasInstanceTypeWitness, similar to what's in JSCallReducer. Also
migrate the {Map,Set}.prototype.size getter inlining to the
JSBuiltinReducer, so that everything is in a single place.

R=jgruber@chromium.org

Bug: v8:6344, v8:6571, chromium:740122
Change-Id: I09cb506fe26ed3e10d7dcb2f95ec4415e639582d
Reviewed-on: https://chromium-review.googlesource.com/570159Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46655}