amd64 front end: accept XACQUIRE and XRELEASE on exactly the insns that
the Intel docs say can validly have them.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2787 8f6e269a-dfd6-0310-a8e1-e2731360e62c
Tidyup -- no functional change.  Replace all "pfx & PFX_LOCK"
with haveLOCK(pfx).


git-svn-id: svn://svn.valgrind.org/vex/trunk@2788 8f6e269a-dfd6-0310-a8e1-e2731360e62c