[AArch64] Enable VECT_COMPARE_COSTS by default for SVE

[AArch64] Enable VECT_COMPARE_COSTS by default for SVE

This patch enables VECT_COMPARE_COSTS by default for SVE, both so
that we can compare SVE against Advanced SIMD and so that (with future
patches) we can compare multiple SVE vectorisation approaches against
each other.  It also adds a target-specific --param to control this.

2019-11-16  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64.opt (--param=aarch64-sve-compare-costs):
	New option.
	* doc/invoke.texi: Document it.
	* config/aarch64/aarch64.c (aarch64_autovectorize_vector_modes):
	By default, return VECT_COMPARE_COSTS for SVE.

gcc/testsuite/
	* gcc.target/aarch64/sve/reduc_3.c: Split multi-vector cases out
	into...
	* gcc.target/aarch64/sve/reduc_3_costly.c: ...this new test,
	passing -fno-vect-cost-model for them.
	* gcc.target/aarch64/sve/slp_6.c: Add -fno-vect-cost-model.
	* gcc.target/aarch64/sve/slp_7.c,
	* gcc.target/aarch64/sve/slp_7_run.c: Split multi-vector cases out
	into...
	* gcc.target/aarch64/sve/slp_7_costly.c,
	* gcc.target/aarch64/sve/slp_7_costly_run.c: ...these new tests,
	passing -fno-vect-cost-model for them.
	* gcc.target/aarch64/sve/while_7.c: Add -fno-vect-cost-model.
	* gcc.target/aarch64/sve/while_9.c: Likewise.

From-SVN: r278337