AArch64: Cleanup move immediate code

AArch64: Cleanup move immediate code

Simplify, refactor and improve various move immediate functions.
Allow 32-bit MOVN/I as a valid 64-bit immediate which removes special
cases in aarch64_internal_mov_immediate.  Add new constraint so the movdi
pattern only needs a single alternative for move immediate.

gcc/
	* config/aarch64/aarch64.cc (aarch64_bitmask_imm): Use unsigned type.
	(aarch64_is_mov_xn_imm): New function.
	(aarch64_move_imm): Refactor, assert mode is SImode or DImode.
	(aarch64_internal_mov_immediate): Assert mode is SImode or DImode.
	Simplify special cases.
	(aarch64_uimm12_shift): Simplify code.
	(aarch64_clamp_to_uimm12_shift): Likewise.
	(aarch64_movw_imm): Rename to aarch64_is_movz.
	(aarch64_float_const_rtx_p): Pass either SImode or DImode to
	aarch64_internal_mov_immediate.
	(aarch64_rtx_costs): Likewise.
	* config/aarch64/aarch64.md (movdi_aarch64): Merge 'N' and 'M'
	constraints into single 'O'.
	(mov<mode>_aarch64): Likewise.
	* config/aarch64/aarch64-protos.h (aarch64_move_imm): Use unsigned.
	(aarch64_bitmask_imm): Likewise.
	(aarch64_uimm12_shift): Likewise.
	(aarch64_is_mov_xn_imm): New prototype.
	* config/aarch64/constraints.md: Add 'O' for 32/64-bit immediates,
	limit 'N' to 64-bit only moves.