This avoids generation of duplicate strings. Especially debug builds
suffer from this and generate 16000+ strings, mostly for CSA_ASSERT
and CAST statements. Deduplicating these trims that down to roughly
1000 strings.

Release builds are affected at a smaller scale. There, we have roughly
100 duplicate strings in the snapshot.

Bug: v8:6666
Change-Id: I688d3b97431b8cea1e98983eab5f07278dae91a0
Reviewed-on: https://chromium-review.googlesource.com/931041Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51472}