Fixed race condition in HBranch::observed_input_representation.

Non-numeric static local variables are not thread-safe, and the case
at hand was a premature optimization anyway: The generated code for
the check in question is exactly the same when unfolded.

BUG=chromium:420483

Review URL: https://codereview.chromium.org/756903003

Cr-Commit-Position: refs/heads/master@{#25502}
harmony-scoping: Catch variable should be VAR, not LET

R=rossberg@chromium.org
BUG=v8:2858
LOG=N

Review URL: https://codereview.chromium.org/748113003

Cr-Commit-Position: refs/heads/master@{#25503}