Subzero: Enhance the timer dump format.

This adds update counts to the output, e.g.:

Total across all functions - Flat times:
    0.262297 (13.0%): [ 1287] linearScan
    0.243965 (12.1%): [ 1287] emit
...

This is useful to know when some passes are called once per function and others are called several times per function.

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/655563005