I'd noticed that loc_mentioned_in_p() wouldn't find loc if it was
mentioned as one of the XVECEXPs of say a concatn, a parallel or
somesuch.  Debugging this, I realized this function would access every
fld of an RTX as if it was a pointer to an RTX itself.  This is wrong:
it makes room for false positives and it accesses as pointers fields
that might have been initialized as narrower or otherwise non-pointer
values, thus invoking undefined behavior.

This patch, that I've already tested in the vta branch, and I'm not
re-testing in mainline (x86_64-linux-gnu for both), should fix this
problem.  Ok to install?