[Bug target/70021] [6 Regression] Test miscompiled with -O3 option for -march=core-avx2.

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70021

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 37834
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=37834&action=edit
gcc6-pr70021-wip.patch

But that would mean we don't vectorize the loop.
I see 2 cases here, one where we look up the def_stmt and look through it, that
is the same case as in the other PR and we shouldn't do that, otherwise we
reference something that is not the final SSA_NAME of a pattern sequence
outside of that sequence.
The other case is where we don't look through that or just avoid that, in that
case we add a cast into the current pattern sequence, and refer to the final
SSA_NAME of the other pattern sequence.  I really don't see why that shouldn't
work right.  Except as the patch shows, it doesn't for some reason.