i386-opts.h (enum prefer_vector_width): Added new enum for the new option -mprefer-vector-width=[none|128|256|512].

	* config/i386/i386-opts.h (enum prefer_vector_width): Added new enum
	for the new option -mprefer-vector-width=[none|128|256|512].
	* config/i386/i386.c (ix86_target_string): remove old style options
	-mprefer-avx256 and make -mprefer-avx128 as alias.
	(ix86_option_override_internal):  Apply defaults for the
	-mprefer-vector-width=[128|256] option.
	* config/i386/i386.h (TARGET_PREFER_AVX128, TARGET_PREFER_AVX256):
	Implement macros to work with -mprefer-vector-width=.
	* config/i386/i386.opt: Implemented option
	-mprefer-vector-width=[none|128|256|512].
	* doc/invoke.texi: Documentation for
	-mprefer-vector-width=[none|128|256|512].

gcc/testsuite/

	* g++.dg/ext/pr57362.C (__attribute__): Test
	prefer-vector-width=[128|256] target attribute.
	* gcc.target/i386/avx512f-constant-float-return.c (dg-optioins):
	Use -mprefer-vector-width=256 instead of -mprefer-avx256.
	* gcc.target/i386/avx512f-prefer.c: Ditto.
	* gcc.target/i386/pr82460-2.c: Ditto.

From-SVN: r255030