[Bug tree-optimization/97678] [11 regression] execution fail for gcc.target/powerpc/p9-vec-length-epil-run-5.c after r11-4614

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97678

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Ah, the vectorized epilogue is wrong since we do not represent the initial
value there correctly.  We still have the {0, 0} invariant in the corresponding
SLP tree.

Hmm.

So previously we're relying on the actual PHI node(s) preheader value plus
eventually honoring LOOP_VINFO_MASK_SKIP_NITERS.