Teach the GetProperty builtin how to perform [[Get]] on JSProxy
instances by calling into the dedicated ProxyGetProperty builtin
that we already use for the LOAD_IC / KEYED_LOAD_IC. This is
important when proxies are used in places were GetProperty builtin
is used like for example as iterables in for..of loops or in spreads.

On a simple micro-benchmark like the following

```js
const proxy = new Proxy([1, 2, 3], {
  get(target, property) { return target[property]; }
});
const TESTS = [
    function testForOfProxy() { for (const x of proxy) {} },
    function testSpreadProxy() { return [...proxy]; }
];

function test(fn) {
  var result;
  for (var i = 0; i < 1e6; ++i) result = fn();
  return result;
}
test(x => x);

for (var j = 0; j < TESTS.length; ++j) test(TESTS[j]);
for (var j = 0; j < TESTS.length; ++j) {
  var startTime = Date.now();
  test(TESTS[j]);
  print(TESTS[j].name + ':', (Date.now() - startTime), 'ms.');
}
```

improves from around

  testForOfProxy: 1672.6 ms.
  testSpreadProxy: 1956.6 ms.

to

  testForOfProxy: 408.4 ms.
  testSpreadProxy: 530.8 ms.

on average, which corresponds to a 4-5x performance improvement, even
for small arrays. On the ARES-6 Air benchmark this completely eliminates
all calls to the %GetProperty runtime function, and thereby improves the
steady state mean by 2-3%.

Bug: v8:6344, v8:6557, v8:6559
Change-Id: Ifebdaff8f3ae5899a33ce408ecd54655247f3a02
Reviewed-on: https://chromium-review.googlesource.com/1199023Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55539}