[binutils][aarch64] New sve_size_bh iclass.

[binutils][aarch64] New sve_size_bh iclass.

Add new iclass sve_size_bh to handle instructions that have two variants
encoded with the SVE_sz field.
This iclass behaves the same as the sve_size_sd iclass, but it has a
nicer name for those instructions that choose between variants using the
"B" and "H" size qualifiers.

include/ChangeLog:

2019-05-09  Matthew Malcomson  <matthew.malcomson@arm.com>

	* opcode/aarch64.h (enum aarch64_insn_class): Add sve_size_bh iclass.

opcodes/ChangeLog:

2019-05-09  Matthew Malcomson  <matthew.malcomson@arm.com>

	* aarch64-asm.c (aarch64_encode_variant_using_iclass): Handle
	sve_size_bh iclass encode.
	* aarch64-dis.c (aarch64_decode_variant_using_iclass): Handle
	sve_size_bh iclass decode.