Normally, the FakeUse for preserving the atomic load ends
up on the load's Dest. However, for fused load+add, the load
is deleted, and its Dest is no longer defined. This trips
up the liveness analysis when it happens on a non-entry
block. So the FakeUse should be for the add's dest instead,
in that case.

We have no access to the add, so introduce a
getLastInserted() helper. A couple of ways to do that:
- modify insert() to track explicitly
- rewind from Next one step

Either that, or we disable the fusing for atomic loads.

BUG=  https://code.google.com/p/nativeclient/issues/detail?id=3882
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/417353003