Merged r14481 into 3.17 branch.

Fix beyond-heap load on x64 Crankshafted StringCharFromCode

BUG=chromium:235311
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/14959008

git-svn-id: https://v8.googlecode.com/svn/branches/3.17@14556 ce2b1a6d-e550-0410-aec6-3dcde31c8c00