- Add a new bytecode for the ToString operation, replacing the old
intrinsic call (currently does not collect type feedback).
- Add a new AST node to represent TemplateLiterals, and avoid
generating unnecessary ToString operations in some simple cases.
- Use a single feedback slot for each string addition, because the
type feedback should always be the same for each addition

This seems to produce a very slight improvement on JSTests benchmarks
and bench-ruben.js from v8:7415, and it's possible that type feedback
for the ToString bytecode could provide more opportunities to eliminate
the runtime call in TurboFan.

Doesn't touch tagged templates

[esnext] fix OOB read in ASTPrinter::VisistTemplateLiteral

Fixes an error where TemplateLiteral printing in --print-ast
would try to read an element beyond the length of a vector.

BUG=v8:7415, chromium:820596
R=adamk@chromium.org, gsathya@chromum.org, rmcilroy@chromium.org, ishell@chromium.org, bmeurer@chromium.org

Change-Id: Ie56894f73a6445550a5f95f42160c4e29ab1da42
Reviewed-on: https://chromium-review.googlesource.com/958408Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Cr-Commit-Position: refs/heads/master@{#51933}