[PATCH, i386] Enable option -mprefer-avx256 as default for Intel Skylake configuration

[Shalnov, Sergey]

Hi,
This patch makes "prefer-avx256" option as default tuning for "skylake-avx512".
This is due to better performance of 256-bit code for some of the cases. In case of 
Skylake Server the Optimization Manual has following "Since port 0 and port 1 are 256-bits wide, 
Intel AVX-512 operations that will be dispatched to port 0 will execute on both port 0 and port 1" 
(chapter 2.1 in https://software.intel.com/sites/default/files/managed/9e/bc/64-ia-32-architectures-optimization-manual.pdf).

It is platform and case specific and not applicable to every AVX512-available platform. 
Option purpose is to make this code generation more performance efficient. It provides 
performance gain on SPEC2017int on Skylake Server for -march=skylake-avx512. 
Patch leaves opportunity for some of AVX-512 instructions generated where they are the most efficient.

2017-10-31  Sergey Shalnov  <Sergey.Shalnov@intel.com>
gcc/
	* config/i386/x86-tune.def (X86_TUNE_AVX256_OPTIMAL): Add tuning
	option prefer-avx256 for skylake-avx512 configuration.
	* config/i386/i386.c (ix86_option_override_internal): Ditto.
	(get_builtin_code_for_version): Ditto.

Sergey

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