[Bug middle-end/28367] accessing via union on a vector does not cause vec_extract to be used

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=28367

--- Comment #7 from Richard Biener <rguenth at gcc dot gnu.org> ---
Yeah, something along that line would be needed.  The issue is that
the partial value is not "available" (as in: it doesn't have a SSA name)
and thus there is nothing to CSE the load to.

I suppose that we should generalize this support for real-/imag-part of
complex values as well.  Not sure if it is worth using BIT_FIELD_REF
for getting at pieces of scalar registers (maybe it would pay off for
targets that can efficiently compute lowpart subregs).