- 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

BUG=v8:7415
R=rmcilroy@chromium.org, ishell@chromium.org, bmeurer@chromium.org

Change-Id: If5a8c68558431f058db894d65776324abf54218e
Reviewed-on: https://chromium-review.googlesource.com/945408Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Cr-Commit-Position: refs/heads/master@{#51853}