[PATCH, i386] Enable option -mprefer-avx256 added for Intel AVX512 configuration

[Jakub Jelinek]

On Thu, Sep 14, 2017 at 12:10:50PM +0000, Shalnov, Sergey wrote:
> GCC has the option "mprefer-avx128" to use 128-bit AVX registers instead
> of 256-bit AVX registers in the auto-vectorizer.

> This patch enables the command line option "mprefer-avx256" that reduces
> 512-bit registers usage in "march=skylake-avx512" mode.  This is the
> initial implementation of the option.  Currently, 512-bit registers might
> appears in some cases.  I have a plan to continue fix the cases where
> 512-bit registers are appear.  Sergey

What is the rationale for this?  -mprefer-avx128 has been added because some
(older) AMD CPUs implement AVX by performing 256-bit ops as two 128-bit uops
and thus it is faster to emit 128-bit only code.
Is that the case for any AVX512 implementations too?

	Jakub