This optimization addresses a case triggered by the Templates/Untagged
js-perf-test, in which template literals get compiled to a cascade of
JSAdd(JSAdd, JSToString) nodes.

Before the optimization in https://chromium-review.googlesource.com/c/v8/v8/+/1193342
JSToString no-ops used to get optimized away during typed lowering together
with constant folding of string concatenation. This change allows us to get
rid of a no-op JSToString call during native context specialization, thus
allowing for constant folding of JSAdd to kick in and CheckStringAdd nodes to
not be generated at all.

This change also removes the NumberToString optimization from typed
lowering, as it's being executed during earlier stage.

Bug: chromium:879083
Change-Id: I1d8155ed969b6959fbb86fca21e4714b88a2695a
Reviewed-on: https://chromium-review.googlesource.com/1202622
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55724}