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

Review URL: https://codereview.chromium.org/1420573002

Cr-Commit-Position: refs/heads/master@{#31437}