s390: Simplify the optimisation for PUTs to tracked registers.

s390: Simplify the optimisation for PUTs to tracked registers.

There is no need to treat the guest IA register specially. The first
assignment in a SB to any tracked register will always assign the full value
and not do an incremental update. Therefore, maintaining
env->first_IA_assignment is redundant. Interestingly, the old code
forgot to mark the tracking state as valid when first_IA_assignment was
true, resulting in an inefficiency (in the order of 0.8% of insns).

Also, when a tracked register already contains the value that it is
being assigned, there is not need to
(a) check whether that register requires precise memory exceptions and
(b) if so, assign the new value.
By definition, the register is up-to-date and reassigning the new value
which is also the old value is meaningless. No idea, what I was thinking
at the time.

git-svn-id: svn://svn.valgrind.org/vex/trunk@2614