Port r21684 (ea35030)

Original commit message:
Patch from Steven Keuchel <keuchel@chromium.org>

BUG=v8:2198
LOG=N
TEST=mjsunit/harmony/block-let-crankshaft.js
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/313183007

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21701 ce2b1a6d-e550-0410-aec6-3dcde31c8c00