Enable auto-vectorization at O2 with very-cheap cost model.

Enable auto-vectorization at O2 with very-cheap cost model.

gcc/ChangeLog:

	* common.opt (ftree-vectorize): Add Var(flag_tree_vectorize).
	* doc/invoke.texi (Options That Control Optimization): Update
	documents.
	* opts.c (default_options_table): Enable auto-vectorization at
	O2 with very-cheap cost model.
	(finish_options): Use cheap cost model for
	explicit -ftree{,-loop}-vectorize.

gcc/testsuite/ChangeLog:

	* c-c++-common/Wstringop-overflow-2.c: Adjust testcase.
	* g++.dg/tree-ssa/pr81408.C: Ditto.
	* g++.dg/warn/Wuninitialized-13.C: Ditto.
	* gcc.dg/Warray-bounds-51.c: Ditto.
	* gcc.dg/Warray-parameter-3.c: Ditto.
	* gcc.dg/Wstringop-overflow-14.c: Ditto.
	* gcc.dg/Wstringop-overflow-21.c: Ditto.
	* gcc.dg/Wstringop-overflow-68.c: Ditto.
	* gcc.dg/Wstringop-overflow-76.c: Ditto.
	* gcc.dg/gomp/pr46032-2.c: Ditto.
	* gcc.dg/gomp/pr46032-3.c: Ditto.
	* gcc.dg/gomp/simd-2.c: Ditto.
	* gcc.dg/gomp/simd-3.c: Ditto.
	* gcc.dg/graphite/fuse-1.c: Ditto.
	* gcc.dg/pr67089-6.c: Ditto.
	* gcc.dg/pr82929-2.c: Ditto.
	* gcc.dg/pr82929.c: Ditto.
	* gcc.dg/store_merging_1.c: Ditto.
	* gcc.dg/store_merging_11.c: Ditto.
	* gcc.dg/store_merging_13.c: Ditto.
	* gcc.dg/store_merging_15.c: Ditto.
	* gcc.dg/store_merging_16.c: Ditto.
	* gcc.dg/store_merging_19.c: Ditto.
	* gcc.dg/store_merging_24.c: Ditto.
	* gcc.dg/store_merging_25.c: Ditto.
	* gcc.dg/store_merging_28.c: Ditto.
	* gcc.dg/store_merging_30.c: Ditto.
	* gcc.dg/store_merging_5.c: Ditto.
	* gcc.dg/store_merging_7.c: Ditto.
	* gcc.dg/store_merging_8.c: Ditto.
	* gcc.dg/strlenopt-85.c: Ditto.
	* gcc.dg/tree-ssa/dump-6.c: Ditto.
	* gcc.dg/tree-ssa/pr19210-1.c: Ditto.
	* gcc.dg/tree-ssa/pr47059.c: Ditto.
	* gcc.dg/tree-ssa/pr86017.c: Ditto.
	* gcc.dg/tree-ssa/pr91482.c: Ditto.
	* gcc.dg/tree-ssa/predcom-1.c: Ditto.
	* gcc.dg/tree-ssa/predcom-dse-3.c: Ditto.
	* gcc.dg/tree-ssa/prefetch-3.c: Ditto.
	* gcc.dg/tree-ssa/prefetch-6.c: Ditto.
	* gcc.dg/tree-ssa/prefetch-8.c: Ditto.
	* gcc.dg/tree-ssa/prefetch-9.c: Ditto.
	* gcc.dg/tree-ssa/ssa-dse-18.c: Ditto.
	* gcc.dg/tree-ssa/ssa-dse-19.c: Ditto.
	* gcc.dg/uninit-40.c: Ditto.
	* gcc.dg/unroll-7.c: Ditto.
	* gcc.misc-tests/help.exp: Ditto.
	* gcc.target/i386/avx512vpopcntdqvl-vpopcntd-1.c: Ditto.
	* gcc.target/i386/pr34012.c: Ditto.
	* gcc.target/i386/pr49781-1.c: Ditto.
	* gcc.target/i386/pr95798-1.c: Ditto.
	* gcc.target/i386/pr95798-2.c: Ditto.
	* gfortran.dg/pr77498.f: Ditto.