Implement --vex-iropt-register-updates=sp-at-mem-access

Implement --vex-iropt-register-updates=sp-at-mem-access

Option sp-at-mem-access can be used by tools which do not
need an up to date BP and IP at each mem access.
It is needed however to have SP up to date at each memory
access, as an up to date SP is needed to grow the stack in
m_signals.c

Tools massif, cachegrind and callgrind are using sp-at-mem-access
as default.
None tool could also use sp-at-mem-access but default is kept
to unwindregs-at-mem-access (similar to memcheck, drd, helgrind, exp-sgcheck).

exp-dhat, exp-bbv, lackey have not been looked at to see if they
could make use of sp-at-mem-access.

Validated on x86, amd64, ppc64 and s390x.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@12872 a5019735-40e9-0310-863c-91ae7b9d1cf9