[Bug tree-optimization/63185] Improve DSE with branches

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63185

--- Comment #7 from Marc Glisse <glisse at gcc dot gnu.org> ---
This PR is messy. To sum up, comment #0 was recently fixed, comment #5 is not
(not noticing that the writes in the loop are dead), and comment #6 asks for
increasing the alignment of VLAs the same way we sometimes increase the
alignment of arrays to help vectorization.