This is purely for convenience of personal testing/debugging.

To demonstrate its correctness in this CL, -build-on-read=0 is removed
from the two .ll lit tests that explicitly use it, and also from the
crosstest.py script.  The lit test wrapper run-llvm2ice.py is left
unchanged to be safe.

BUG= none
R=kschimpf@google.com

Review URL: https://codereview.chromium.org/732583002