Fix an earlier regression which forbid non-VariableProxy LHS from being
used in for-of loops. Like for-in loops, the spec allows any LHS to be used,
with the sole exception that ObjectLiterals and ArrayLiterals must be valid
AssignmentPatterns.

Also fixes a bug in TurboFan which resulted in incorrectly replacing a variable load with a constant value in some instances, due to the AstLoopAssignmentAnalyzer failing to record the assignment to ForOfStatement's value.

BUG=v8:4418, v8:2720
LOG=N
R=wingo@igalia.com, littledan@chromium.org, adamk@chromium.org, bmeurer@chromium.org

Review URL: https://codereview.chromium.org/1411873004

Cr-Commit-Position: refs/heads/master@{#31816}