[Bug tree-optimization/54346] combine permutations

[glisse at gcc dot gnu.org]

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

--- Comment #6 from Marc Glisse <glisse at gcc dot gnu.org> ---
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?