[Bug tree-optimization/58817] optimize alloca with constant size

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58817

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
We do this transform in fold_builtin_alloca_with_align (), I'm not sure but
maybe it does not happen because 'a' escapes to f.