aarch64: Add svboolx2_t

aarch64: Add svboolx2_t

SME2 has some instructions that operate on pairs of predicates.
The SME2 ACLE defines an svboolx2_t type for the associated
intrinsics.

The patch uses a double-width predicate mode, VNx32BI, to represent
the contents, similarly to how data vector tuples work.  At present
there doesn't seem to be any need to define pairs for VNx2BI,
VNx4BI and VNx8BI.

We already supported pairs of svbool_ts at the PCS level, as part
of a more general framework.  All that changes on the PCS side is
that we now have an associated mode.

gcc/
	* config/aarch64/aarch64-modes.def (VNx32BI): New mode.
	* config/aarch64/aarch64-protos.h (aarch64_split_double_move): Declare.
	* config/aarch64/aarch64-sve-builtins.cc
	(register_tuple_type): Handle tuples of predicates.
	(handle_arm_sve_h): Define svboolx2_t as a pair of two svbool_ts.
	* config/aarch64/aarch64-sve.md (movvnx32bi): New insn.
	* config/aarch64/aarch64.cc
	(pure_scalable_type_info::piece::get_rtx): Use VNx32BI for pairs
	of predicates.
	(pure_scalable_type_info::add_piece): Don't try to form pairs of
	predicates.
	(VEC_STRUCT): Generalize comment.
	(aarch64_classify_vector_mode): Handle VNx32BI.
	(aarch64_array_mode): Likewise.  Return BLKmode for arrays of
	predicates that have no associated mode, rather than allowing
	an integer mode to be chosen.
	(aarch64_hard_regno_nregs): Handle VNx32BI.
	(aarch64_hard_regno_mode_ok): Likewise.
	(aarch64_split_double_move): New function, split out from...
	(aarch64_split_128bit_move): ...here.
	(aarch64_ptrue_reg): Tighten assert to aarch64_sve_pred_mode_p.
	(aarch64_pfalse_reg): Likewise.
	(aarch64_sve_same_pred_for_ptest_p): Likewise.
	(aarch64_sme_mode_switch_regs::add_reg): Handle VNx32BI.
	(aarch64_expand_mov_immediate): Restrict handling of boolean vector
	constants to single-predicate modes.
	(aarch64_classify_address): Handle VNx32BI, ensuring that both halves
	can be addressed.
	(aarch64_class_max_nregs): Handle VNx32BI.
	(aarch64_member_type_forces_blk): Don't for BLKmode for svboolx2_t.
	(aarch64_simd_valid_immediate): Allow all-zeros and all-ones for
	VNx32BI.
	(aarch64_mov_operand_p): Restrict predicate constant canonicalization
	to single-predicate modes.
	(aarch64_evpc_ext): Generalize exclusion to all predicate modes.
	(aarch64_evpc_rev_local, aarch64_evpc_dup): Likewise.
	* config/aarch64/constraints.md (PR_REGS): New predicate.

gcc/testsuite/
	* gcc.target/aarch64/sve/pcs/struct_3_128.c (test_nonpst3): Adjust
	stack offsets.
	(ret_nonpst3): Remove XFAIL.
	* gcc.target/aarch64/sve/acle/general-c/svboolx2_1.c: New test.