When lowering of a couple of atomic intrinsics down to a loop structure, a FakeUse on the memory address's base variable is created.  However, if the memory address is a global constant, there is no base variable.  So check for that and don't create a FakeUse if there is none.

BUG= none
TEST=synchronization_sync (scons test)
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/1023673007