ppc64: lxvw4x instruction uses four 32-byte loads. When run on an

ppc64:  lxvw4x instruction uses four 32-byte loads.  When run on an
application that does partial loads an error message is generated by
valgrind about Invalid read of size 4.  Valgrind is incorrectly
detecting the invalid read.  The four loads were replaced
by a single 128-bit load.  The invalid read message can now be
suppressed using the command line option " --partial-loads-ok=yes ".

This fix is for Valgrind bugzilla 339433.

git-svn-id: svn://svn.valgrind.org/vex/trunk@2962