aarch64: Improve on ldp-stp policies code structure.

aarch64: Improve on ldp-stp policies code structure.

Improves on: 834fc2bf

This improves the code structure of the ldp-stp policies
patch introduced in 834fc2bf

Bootstrapped and regtested on aarch64-linux.

gcc/ChangeLog:
	* config/aarch64/aarch64-opts.h (enum aarch64_ldp_policy): Removed.
	(enum aarch64_ldp_stp_policy): Merged enums aarch64_ldp_policy
	and aarch64_stp_policy to aarch64_ldp_stp_policy.
	(enum aarch64_stp_policy): Removed.
	* config/aarch64/aarch64-protos.h (struct tune_params): Removed
	aarch64_ldp_policy_model and aarch64_stp_policy_model enum types
	and left only the definitions to the aarch64-opts one.
	* config/aarch64/aarch64.cc (aarch64_parse_ldp_policy): Removed.
	(aarch64_parse_stp_policy): Removed.
	(aarch64_override_options_internal): Removed calls to parsing
	functions and added obvious direct assignments.
	(aarch64_mem_ok_with_ldpstp_policy_model): Improved
	code quality based on the new changes.
	* config/aarch64/aarch64.opt: Use single enum type
	aarch64_ldp_stp_policy for both ldp and stp options.

gcc/testsuite/ChangeLog:
	* gcc.target/aarch64/ldp_aligned.c: Splitted into this and
	ldp_unaligned.
	* gcc.target/aarch64/stp_aligned.c: Splitted into this and
	stp_unaligned.
	* gcc.target/aarch64/ldp_unaligned.c: New test.
	* gcc.target/aarch64/stp_unaligned.c: New test.

Signed-off-by: Manos Anagnostakis <manos.anagnostakis@vrull.eu>
Suggested-by: Richard Sandiford <richard.sandiford@arm.com>