[turbofan] Fix liveness analysis for let variable in TDZ.

[turbofan] Fix liveness analysis for let variable in TDZ.

This makes sure that initializing assignments of let-declared variables
perform an environment lookup and hence keep the variable alive. This is
needed because full-codegen contains debug code verifying the variable
is still inside the TDZ at the initializing assignment.

R=jarin@chromium.org
TEST=mjsunit/compiler/regress-variable-liveness-let
BUG=v8:4493
LOG=n

Committed: https://crrev.com/d9a5add017d732ee9a536f85fe7abd9450f99215
Cr-Commit-Position: refs/heads/master@{#31437}

[Michael Starzinger, 2015-10-20 16:56:03]

Description was changed from

==========
[turbofan] Fix liveness analysis for let variable in TDZ.

This makes sure that initializing assignments of let-declared variables
perform an evironment lookup and hence keep the varianle alive. This is
needed because full-codegen contains debug code verifying the variable
is still inside the TDZ at the initializing assignment.

R=jarin@chromium.org
TEST=mjsunit/compiler/regress-variable-liveness-let
BUG=v8:4493
LOG=n
==========

to

==========
[turbofan] Fix liveness analysis for let variable in TDZ.

This makes sure that initializing assignments of let-declared variables
perform an environment lookup and hence keep the variable alive. This is
needed because full-codegen contains debug code verifying the variable
is still inside the TDZ at the initializing assignment.

R=jarin@chromium.org
TEST=mjsunit/compiler/regress-variable-liveness-let
BUG=v8:4493
LOG=n
==========

[Jarin, 2015-10-21 11:23:24]

lgtm

[Michael Starzinger, 2015-10-21 12:21:20]

The CQ bit was checked by mstarzinger@chromium.org

[commit-bot: I haz the power, 2015-10-21 12:21:25]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1420573002/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1420573002/40001

[commit-bot: I haz the power, 2015-10-21 12:22:53]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-10-21 12:23:12]

Patchset 3 (id:??) landed as
https://crrev.com/d9a5add017d732ee9a536f85fe7abd9450f99215
Cr-Commit-Position: refs/heads/master@{#31437}