[Bug tree-optimization/63406] -Warray-bounds false positive with -O3

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63406

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
I think we have similar dups elsewhere (warning for last unrolled iteration
which is never executed).

My previous naiive attempts dropped warnings from VRP2 and only warn from
VRP1 (but that regresses some cases in the testsuite).