aarch64: Add backend support for DFP

aarch64: Add backend support for DFP

This patch updates the aarch64 backend as needed to support DFP modes
(SD, DD and TD).

Changes	v1->v2:

* Drop	support	for DFP	modes in
  aarch64_gen_{load||store}[wb]_pair as these are only used in
  prologue/epilogue where DFP modes are not used.  Drop	the
  changes to the corresponding patterns in aarch64.md, and
  useless GPF_PAIR iterator.

* In aarch64_reinterpret_float_as_int, handle DDmode the same way
  as DFmode (needed in case the representation of the
  floating-point value can be loaded using mov/movk.

* In aarch64_float_const_zero_rtx_p, reject constants with DFP
  mode: when X is zero, the callers want to emit either '0' or
  'zr' depending on the context, which is not the way 0.0 is
  represented in DFP mode (in particular fmov d0, #0 is not right
  for DFP).

* In aarch64_legitimate_constant_p, accept DFP

2022-03-31  Christophe Lyon  <christophe.lyon@arm.com>

	gcc/
	* config/aarch64/aarch64.cc
	(aarch64_split_128bit_move): Handle DFP modes.
	(aarch64_mode_valid_for_sched_fusion_p): Likewise.
	(aarch64_classify_address): Likewise.
	(aarch64_legitimize_address_displacement): Likewise.
	(aarch64_reinterpret_float_as_int): Likewise.
	(aarch64_float_const_zero_rtx_p): Likewise.
	(aarch64_can_const_movi_rtx_p): Likewise.
	(aarch64_anchor_offset): Likewise.
	(aarch64_secondary_reload): Likewise.
	(aarch64_rtx_costs): Likewise.
	(aarch64_legitimate_constant_p): Likewise.
	(aarch64_gimplify_va_arg_expr): Likewise.
	(aapcs_vfp_sub_candidate): Likewise.
	(aarch64_vfp_is_call_or_return_candidate): Likewise.
	(aarch64_output_scalar_simd_mov_immediate): Likewise.
	(aarch64_gen_adjusted_ldpstp): Likewise.
	(aarch64_scalar_mode_supported_p): Accept DFP modes if enabled.
	* config/aarch64/aarch64.md
	(movsf_aarch64): Use SFD iterator and rename into
	mov<mode>_aarch64.
	(movdf_aarch64): Use DFD iterator and rename into
	mov<mode>_aarch64.
	(movtf_aarch64): Use TFD iterator and rename into
	mov<mode>_aarch64.
	(split pattern for move TF mode): Use TFD iterator.
	* config/aarch64/iterators.md
	(GPF_TF_F16_MOV): Add DFP modes.
	(SFD, DFD, TFD): New iterators.
	(GPF_TF): Add DFP modes.
	(TX, DX, DX2): Likewise.