This is in preparation of adding a dedicated StringLength operator that
loads the string length. This way operations on strings don't sit in the
effect chain anymore until the EffectControlLinearizer, which wires them.

The NewConsString semantics could still be better, i.e. it could try to
figure out the proper map instead of going for the CONS_STRING_TYPE
always. But this change is meant to be just about pushing the logic down
to the EffectControlLinearizer, which we didn't have initially when the
ConsString handling was done.

This also allows us to remove the handling of CONS_STRING_TYPE from the
Deoptimizer, since the escape analysis no longer sees cons strings.

Bug: v8:5269, v8:6936, v8:7109, v8:7137
Change-Id: If6c4a6d7cf63a3a3f7a34a920c8e50a94dfa67fa
Reviewed-on: https://chromium-review.googlesource.com/796413
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49729}