In Promise.all we used to allocate a fresh closure plus a fresh context
for each individual element, which is quite a lot of overhead, especially
since this could be shared in a single context for all elements. The only
bit of information that is needed for each resolve element closure is the
index under which to store the resulting value. With this change we move
this index to the "identity hash" field of the JSFunction, which doesn't
care about the concrete value anyways, as long as it's not zero (the "no
hash" sentinel), and share the rest of the fields in a single outer
context for all resolve element closures.

This limits the maximum number of elements for Promise.all to 2^21 for
now, but that should be fine. Shall we ever see the need for more than
this, we can add machinery to overflow to separate context for indices
larger than 2^21.

This significantly reduces the overhead due to Promise.all on the
parallel-async-es2017-native test, with execution time dropping from
around 148ms to 133ms, so overall a steady 10% improvement on this
benchmark.

Bug: v8:7253
Change-Id: I1092da771c4919f3db7129d2b0a244fc26a7b144
Reviewed-on: https://chromium-review.googlesource.com/973283Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52134}