Enabling vectorization at -O2 for x86 generic, core and zen tuning

[Eric Botcazou]

> Note that I benchmarked -ftree-slp-vectorize separately before and
> results was hit/miss, so perhaps enabling only -ftree-vectorize would
> give better compile time tradeoffs. I was worried of partial memory
> stalls, but I will benchmark it and also benchmark difference between
> cost models.

; Alias to enable both -ftree-loop-vectorize and -ftree-slp-vectorize.
ftree-vectorize
Common Report Optimization
Enable vectorization on trees.

-- 
Eric Botcazou