aarch64: Fine-grained policies to control ldp-stp formation

This patch implements the following TODO in gcc/config/aarch64/aarch64.cc
to provide the requested behaviour for handling ldp and stp:

  /* Allow the tuning structure to disable LDP instruction formation
     from combining instructions (e.g., in peephole2).
     TODO: Implement fine-grained tuning control for LDP and STP:
           1. control policies for load and store separately;
           2. support the following policies:
              - default (use what is in the tuning structure)
              - always
              - never
              - aligned (only if the compiler can prove that the
                load will be aligned to 2 * element_size)  */

It provides two new and concrete target-specific command-line parameters
--param=aarch64-ldp-policy= and --param=aarch64-stp-policy=
to give the ability to control load and store policies seperately as
stated in part 1 of the TODO.

The accepted values for both parameters are:
 * default: Use the policy of the tuning structure (default).
 * always: Emit ldp/stp regardless of alignment.
 * never: Do not emit ldp/stp.
 * aligned: In order to emit ldp/stp, first check if the load/store will
   be aligned to 2 * element_size.

Bootstrapped and regtested aarch64-linux.

gcc/ChangeLog:
	* config/aarch64/aarch64-opts.h (enum aarch64_ldp_policy): New
	enum type.
	(enum aarch64_stp_policy): New enum type.
	* config/aarch64/aarch64-protos.h (struct tune_params): Add
	appropriate enums for the policies.
	(aarch64_mem_ok_with_ldpstp_policy_model): New declaration.
	* config/aarch64/aarch64-tuning-flags.def
	(AARCH64_EXTRA_TUNING_OPTION): Remove superseded tuning
	options.
	* config/aarch64/aarch64.cc (aarch64_parse_ldp_policy): New
	function to parse ldp-policy parameter.
	(aarch64_parse_stp_policy): New function to parse stp-policy parameter.
	(aarch64_override_options_internal): Call parsing functions.
	(aarch64_mem_ok_with_ldpstp_policy_model): New function.
	(aarch64_operands_ok_for_ldpstp): Add call to
	aarch64_mem_ok_with_ldpstp_policy_model for parameter-value
	check and alignment check and remove superseded ones.
	(aarch64_operands_adjust_ok_for_ldpstp): Add call to
	aarch64_mem_ok_with_ldpstp_policy_model for parameter-value
	check and alignment check and remove superseded ones.
	* config/aarch64/aarch64.opt (aarch64-ldp-policy): New param.
	(aarch64-stp-policy): New param.
	* doc/invoke.texi: Document the parameters accordingly.

gcc/testsuite/ChangeLog:
	* gcc.target/aarch64/ampere1-no_ldp_combine.c: Removed.
	* gcc.target/aarch64/ldp_aligned.c: New test.
	* gcc.target/aarch64/ldp_always.c: New test.
	* gcc.target/aarch64/ldp_never.c: New test.
	* gcc.target/aarch64/stp_aligned.c: New test.
	* gcc.target/aarch64/stp_always.c: New test.
	* gcc.target/aarch64/stp_never.c: New test.

Signed-off-by: Manos Anagnostakis <manos.anagnostakis@vrull.eu>
Co-Authored-By: Philipp Tomsich <philipp.tomsich@vrull.eu>
Co-Authored-By: Manolis Tsamis <manolis.tsamis@vrull.eu>

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