[Bug tree-optimization/68707] [6 Regression] testcase gcc.dg/vect/O3-pr36098.c vectorized using VEC_PERM_EXPR rather than VEC_LOAD_LANES

[alalaw01 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68707

--- Comment #8 from alalaw01 at gcc dot gnu.org ---
Adding a check against BB SLP avoids some regressions caused by bailing out of
BB SLP when we can't then do a load/store-lanes.