[turbofan] Fix invalid lowering of let variable in TDZ.

[turbofan] Fix invalid lowering of let variable in TDZ.

This fixes JSNativeContextSpecialization to not lower JSLoadGlobal and
JSStoreGlobal nodes if the global variable has morphed into a context
variable that is currently within a TDZ. Scary variable binding is being
scary!

R=bmeurer@chromium.org
TEST=cctest/test-decls/Regress3941 --turbo-filter="f"
BUG=v8:4470
LOG=n

Committed: https://crrev.com/4de969cebe8288a96b77e319f4fce113c979b5d7
Cr-Commit-Position: refs/heads/master@{#31405}

[Benedikt Meurer, 2015-10-20 09:42:36]

LGTM, thanks!

[Michael Starzinger, 2015-10-20 09:48:05]

Description was changed from

==========
[turbofan] Fix invalid lowering of let variable in TDZ.

This fixes JSNativeContextSpecialization to not lower JSLoadGlobal and
JSStoreGlobal nodes if the global variable has morphed into a context
variable that is currently within a TDZ. Scarry variable binding is
being scarry!

R=bmeurer@chromium.org
TEST=cctest/test-decls/Regress3941 --turbo-filter="f"
BUG=v8:4470
LOG=n
==========

to

==========
[turbofan] Fix invalid lowering of let variable in TDZ.

This fixes JSNativeContextSpecialization to not lower JSLoadGlobal and
JSStoreGlobal nodes if the global variable has morphed into a context
variable that is currently within a TDZ. Scary variable binding is being
scary!

R=bmeurer@chromium.org
TEST=cctest/test-decls/Regress3941 --turbo-filter="f"
BUG=v8:4470
LOG=n
==========

[Michael Starzinger, 2015-10-20 10:32:16]

The CQ bit was checked by mstarzinger@chromium.org

[commit-bot: I haz the power, 2015-10-20 10:32:18]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1415733003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1415733003/20001

[commit-bot: I haz the power, 2015-10-20 10:37:30]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-10-20 10:37:52]

Patchset 2 (id:??) landed as
https://crrev.com/4de969cebe8288a96b77e319f4fce113c979b5d7
Cr-Commit-Position: refs/heads/master@{#31405}