A comprehensive test case for 128 bit shadow vector loads in the case
of partial addressibility, for --shadow-loads-ok=yes and =no.  Not
portable and not hooked up to the test/build system yet.

Pertains to #294285.



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13489 a5019735-40e9-0310-863c-91ae7b9d1cf9