[Bug tree-optimization/54346] combine permutations

[crazylht at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54346

--- Comment #7 from Hongtao.liu <crazylht at gmail dot com> ---
(In reply to Marc Glisse from comment #6)
> The log says that this breaks tree-ssa/forwprop-19.c, but I don't see any
> xfail or anything. Does it only fail because gimple-simplify leaves some
> dead code around, so you could update the test to scan the next DCE pass
> dump instead of forwprop1? Or are we missing a transformation that just
> detects a VEC_PERM_EXPR with an identity permutation?

Uoops, I didn't notice this, will add an incremental patch to handle the
indentical index (forwporp-19.c) scenario， sorry.