aarch64: Introduce --param=aarch64-autovec-preference to select autovec preference in backend

This is a patch that introduces the aarch64-autovec-preference that can
take values from 0 - 4, 0 being the default.
It can be used to override the autovectorisation preferences in the
backend:
0 - use default scheme
1 - only use Advanced SIMD
2 - only use SVE
3 - use Advanced SIMD and SVE, prefer Advanced SIMD in the event of a
tie (as determined by costs)
4 - use Advanced SIMD and SVE, prefer SVE in the event of a tie (as
determined by costs)

It can valuable for experimentation when comparing SVE and Advanced SIMD
autovectorisation strategies.

It achieves this adjusting the order of the interleaved SVE and Advanced
SIMD modes in aarch64_autovectorize_vector_modes.
It also adjusts aarch64_preferred_simd_mode to use the new comparison
function to pick Advanced SIMD or SVE to start with.

Bootstrapped and tested on aarch64-none-linux-gnu.

gcc/
	* config/aarch64/aarch64.opt
	(-param=aarch64-autovec-preference): Define.
	* config/aarch64/aarch64.c (aarch64_override_options_internal):
	Set aarch64_sve_compare_costs to 0 when preferring only Advanced
	SIMD.
	(aarch64_cmp_autovec_modes): Define.
	(aarch64_preferred_simd_mode): Adjust to use the above.
	(aarch64_autovectorize_vector_modes): Likewise.
	* doc/invoke.texi: Document aarch64-autovec-preference param.

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