Fix memcheck/tests/vbit-test (the vbit test program) to track changes in bug 387664.

Fix memcheck/tests/vbit-test (the vbit test program) to track changes in bug 387664.

Bug 387664 changes the default settings for accurate definedness checking
for {Add,Sub}{32,64} and {CmpEQ,CmpNE}{8,16,32,64}.  This fix updates the
vbit tester (memcheck/tests/vbit-test) to test the accurate versions of
these, and thereby fixes a regression caused by
e847cb5429927317023d8410c3c56952aa47fb08 as committed for bug 387664.