Align ix86_{move_max,store_max} with vectorizer.

When none of mprefer-vector-width, avx256_optimal/avx128_optimal,
avx256_store_by_pieces/avx512_store_by_pieces is specified, GCC will
set ix86_{move_max,store_max} as max available vector length except
for AVX part.

	      if (TARGET_AVX512F_P (opts->x_ix86_isa_flags)
		  && TARGET_EVEX512_P (opts->x_ix86_isa_flags2))
		opts->x_ix86_move_max = PVW_AVX512;
	      else
		opts->x_ix86_move_max = PVW_AVX128;

So for -mavx2, vectorizer will choose 256-bit for vectorization, but
128-bit is used for struct copy, there could be a potential STLF issue
due to this "misalign".

The patch fixes that.

gcc/ChangeLog:

	* config/i386/i386-options.cc (ix86_option_override_internal):
	set ix86_{move_max,store_max} to PVW_AVX256 when TARGET_AVX
	instead of PVW_AVX128.

gcc/testsuite/ChangeLog:
	* gcc.target/i386/pieces-memcpy-10.c: Add -mprefer-vector-width=128.
	* gcc.target/i386/pieces-memcpy-6.c: Ditto.
	* gcc.target/i386/pieces-memset-38.c: Ditto.
	* gcc.target/i386/pieces-memset-40.c: Ditto.
	* gcc.target/i386/pieces-memset-41.c: Ditto.
	* gcc.target/i386/pieces-memset-42.c: Ditto.
	* gcc.target/i386/pieces-memset-43.c: Ditto.
	* gcc.target/i386/pieces-strcpy-2.c: Ditto.
	* gcc.target/i386/pieces-memcpy-22.c: New test.
	* gcc.target/i386/pieces-memset-51.c: New test.
	* gcc.target/i386/pieces-strcpy-3.c: New test.