bpf: Improvements in CO-RE builtins implementation.

This patch moved the processing of attribute preserve_access_index to
its own independent pass in a gimple lowering pass.
This approach is more consistent with the implementation of the CO-RE
builtins when used explicitly in the code.  The attributed type accesses
are now early converted to __builtin_core_reloc builtin instead of being
kept as an expression in code through out all of the middle-end.
This disables the compiler to optimize out or manipulate the expression
using the local defined type, instead of assuming nothing is known about
this expression, as it should be the case in all of the CO-RE
relocations.

In the process, also the __builtin_preserve_access_index has been
improved to generate code for more complex expressions that would
require more then one CO-RE relocation.
This turned out to be a requirement, since bpf-next selftests would rely on
loop unrolling in order to convert an undefined index array access into a
defined one. This seemed extreme to expect for the unroll to happen, and for
that reason GCC still generates correct code in such scenarios, even when index
access is never predictable or unrolling does not occur.

gcc/ChangeLog:
	* config/bpf/bpf-passes.def (pass_lower_bpf_core): Added pass.
	* config/bpf/bpf-protos.h: Added prototype for new pass.
	* config/bpf/bpf.cc (bpf_delegitimize_address): New function.
	* config/bpf/bpf.md (mov_reloc_core<MM:mode>): Prefixed
	name with '*'.
	* config/bpf/core-builtins.cc (cr_builtins) Added access_node to
	struct.
	(is_attr_preserve_access): Improved check.
	(core_field_info): Make use of root_for_core_field_info
	function.
	(process_field_expr): Adapted to new functions.
	(pack_type): Small improvement.
	(bpf_handle_plugin_finish_type): Adapted to GTY(()).
	(bpf_init_core_builtins): Changed to new function names.
	(construct_builtin_core_reloc): Improved implementation.
	(bpf_resolve_overloaded_core_builtin): Changed how
	__builtin_preserve_access_index is converted.
	(compute_field_expr): Corrected implementation. Added
	access_node argument.
	(bpf_core_get_index): Added valid argument.
	(root_for_core_field_info, pack_field_expr)
	(core_expr_with_field_expr_plus_base, make_core_safe_access_index)
	(replace_core_access_index_comp_expr, maybe_get_base_for_field_expr)
	(core_access_clean, core_is_access_index, core_mark_as_access_index)
	(make_gimple_core_safe_access_index, execute_lower_bpf_core)
	(make_pass_lower_bpf_core): Added functions.
	(pass_data_lower_bpf_core): New pass struct.
	(pass_lower_bpf_core): New gimple_opt_pass class.
	(pack_field_expr_for_preserve_field)
	(bpf_replace_core_move_operands): Removed function.
	(bpf_enum_value_kind): Added GTY(()).
	* config/bpf/core-builtins.h (bpf_field_info_kind, bpf_type_id_kind)
	(bpf_type_info_kind, bpf_enum_value_kind): New enum.
	* config/bpf/t-bpf: Added pass bpf-passes.def to PASSES_EXTRA.

gcc/testsuite/ChangeLog:
	* gcc.target/bpf/core-attr-5.c: New test.
	* gcc.target/bpf/core-attr-6.c: New test.
	* gcc.target/bpf/core-builtin-1.c: Corrected
	* gcc.target/bpf/core-builtin-enumvalue-opt.c: Corrected regular
	expression.
	* gcc.target/bpf/core-builtin-enumvalue.c: Corrected regular
	expression.
	* gcc.target/bpf/core-builtin-exprlist-1.c: New test.
	* gcc.target/bpf/core-builtin-exprlist-2.c: New test.
	* gcc.target/bpf/core-builtin-exprlist-3.c: New test.
	* gcc.target/bpf/core-builtin-exprlist-4.c: New test.
	* gcc.target/bpf/core-builtin-fieldinfo-offset-1.c: Extra tests

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