memory-barriers: Fix description of 2-legged-if-based control dependencies

Sad to say, current compilers really will hoist identical stores from both
branches of an "if" statement to precede the conditional.  This commit
therefore updates the description of control dependencies to reflect this
ugly reality.

Reported-by: Pranith Kumar <bobby.prani@gmail.com>
Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>