Add a new tune avx256_avoid_vec_perm for SRF.

Add a new tune avx256_avoid_vec_perm for SRF.

According to Intel SOM[1], For Crestmont,  most 256-bit Intel AVX2
instructions can be decomposed into two independent 128-bit
micro-operations, except for a subset of Intel AVX2 instructions,
known as cross-lane operations, can only compute the result for an
element by utilizing one or more sources belonging to other elements.

The 256-bit instructions listed below use more operand sources than
can be natively supported by a single reservation station within these
microarchitectures. They are decomposed into two μops, where the first
μop resolves a subset of operand dependencies across two cycles. The
dependent second μop executes the 256-bit operation by using a single
128-bit execution port for two consecutive cycles with a five-cycle
latency for a total latency of seven cycles.

VPERM2I128 ymm1, ymm2, ymm3/m256, imm8
VPERM2F128 ymm1, ymm2, ymm3/m256, imm8
VPERMPD ymm1, ymm2/m256, imm8
VPERMPS ymm1, ymm2, ymm3/m256
VPERMD ymm1, ymm2, ymm3/m256
VPERMQ ymm1, ymm2/m256, imm8

Instead of setting tune avx128_optimal for SRF, the patch add a new
tune avx256_avoid_vec_perm for it. so by default, vectorizer still
uses 256-bit VF if cost is profitable, but lowers to 128-bit whenever
256-bit vec_perm is needed for auto-vectorization. w/o vec_perm,
performance of 256-bit vectorization should be similar as 128-bit
ones(some benchmark results show it's even better than 128-bit
vectorization since it enables more parallelism for convert cases.)

[1] https://www.intel.com/content/www/us/en/content-details/814198/intel-64-and-ia-32-architectures-optimization-reference-manual-volume-1.html

gcc/ChangeLog:

	* config/i386/i386.cc (ix86_vector_costs::ix86_vector_costs):
	Add new member m_num_avx256_vec_perm.
	(ix86_vector_costs::add_stmt_cost): Record 256-bit vec_perm.
	(ix86_vector_costs::finish_cost): Prevent vectorization for
	TAREGT_AVX256_AVOID_VEC_PERM when there's 256-bit vec_perm
	instruction.
	* config/i386/i386.h (TARGET_AVX256_AVOID_VEC_PERM): New
	Macro.
	* config/i386/x86-tune.def (X86_TUNE_AVX256_SPLIT_REGS): Add
	m_CORE_ATOM.
	(X86_TUNE_AVX256_AVOID_VEC_PERM): New tune.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/avx256_avoid_vec_perm.c: New test.