CSA::EmitElementStore used to bail out (IC miss) via
CSA::CheckForCapacityGrow when the capacity hits the new space
limit, causing the store IC to go megamorphic in my example (see
referenced bug). With this CL, we do what TF'ed code does already:
call into Runtime::kGrowArrayElements (in this situation), thus
staying monomorphic.

Here's a contrived test case:

////////////////////////
let x = [];

function bar() {
  for (let i = 0; i < 50000; ++i) x[i] = i;
}

function foo() {
  for (let i = x.length; i < 100e6; ++i) x[i] = i;
}

bar();
foo();
////////////////////////

This took about 4s on my machine, now it takes 3s.

Bug: v8:7447
Change-Id: I7f268fc55835f363d250613ce0357444a663051c
Reviewed-on: https://chromium-review.googlesource.com/918723
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51297}