[AArch64][SVE 08/32] Generalise aarch64_double_precision_fmovable

[AArch64][SVE 08/32] Generalise aarch64_double_precision_fmovable

SVE has single-bit floating-point constants that don't really
have any relation to the AArch64 8-bit floating-point encoding.
(E.g. one of the constants selects between 0 and 1.)  The easiest
way of representing them in the aarch64_opnd_info seemed to be
to use the IEEE float representation directly, rather than invent
some new scheme.

This patch paves the way for that by making the code that converts IEEE
doubles to IEEE floats accept any value in the range of an IEEE float,
not just zero and 8-bit floats.  It leaves the range checking to the
caller (which already handles it).

gas/
	* config/tc-aarch64.c (aarch64_double_precision_fmovable): Rename
	to...
	(can_convert_double_to_float): ...this.  Accept any double-precision
	value that converts to single precision without loss of precision.
	(parse_aarch64_imm_float): Update accordingly.