[PATCH] Assign a low branch weight to ASan's slow path.

[Jonas Wagner]

This patch causes AddressSantitizer to emit branch weights for the branches
leading to its slow path. Experiments on SPEC show speedups of 3-5% on average.
The corresponding discussion on the address-sanitizer mailing list is at
https://groups.google.com/forum/#!topic/address-sanitizer/feaKX15urgQ .

Sorry for the first, badly formatted patch. This one uses git-send-email and
hopefully gets it right.

- Jonas