Allow read side effects for removing dead instructions.

Rationale:
Instructions that only have the harmless read side effect may
be removed when dead as well, we were too strict previously.
As proof of concept, this cl also provides more accurate information
on a few string related intrinsics. This removes the dead indexOf
from CaffeineString (17% performance improvement, big bottleneck
of the StringBuffer's toString() still remains in loop).

Test: test-art-host
Change-Id: Id835a8e287e13e1f09be6b46278a039b8865802e