[gas][aarch64][SVE2] Fix pmull{t,b} requirement on SVE2-AES

[gas][aarch64][SVE2] Fix pmull{t,b} requirement on SVE2-AES

I had mistakenly given all variants of the new SVE2 instructions
pmull{t,b} a dependency on the feature +sve2-aes.

Only the variant specifying .Q -> .D  sizes should have that
restriction.

This patch fixes that mistake and updates the testsuite to have extra
tests (matching the given set of tests per line in aarch64-tbl.h that
the rest of the SVE2 tests follow).

We also add a line in the documentation of the command line to clarify
how to enable `pmull{t,b}` of this larger size.  This is needed because
all other instructions gated under the `sve2-aes` architecture extension
are marked in the instruction documentation by an `HaveSVE2AES` check
while pmull{t,b} is gated under the `HaveSVE2PMULL128` check.

Regtested targeting aarch64-linux.

gas/ChangeLog:

2019-07-01  Matthew Malcomson  <matthew.malcomson@arm.com>

	* testsuite/gas/aarch64/illegal-sve2-aes.d: Update tests.
	* testsuite/gas/aarch64/illegal-sve2.l: Update tests.
	* doc/c-aarch64.texi: Add special note of pmull{t,b}
	instructions under the sve2-aes architecture extension.
	* testsuite/gas/aarch64/illegal-sve2.s: Add small size
	pmull{t,b} instructions.
	* testsuite/gas/aarch64/sve2.d: Add small size pmull{t,b}
	disassembly.
	* testsuite/gas/aarch64/sve2.s: Add small size pmull{t,b}
	instructions.

include/ChangeLog:

2019-07-01  Matthew Malcomson  <matthew.malcomson@arm.com>

	* opcode/aarch64.h (enum aarch64_insn_class): sve_size_013
	renamed to sve_size_13.

opcodes/ChangeLog:

2019-07-01  Matthew Malcomson  <matthew.malcomson@arm.com>

	* aarch64-asm.c (aarch64_encode_variant_using_iclass): Use new
	sve_size_13 icode to account for variant behaviour of
	pmull{t,b}.
	* aarch64-dis-2.c: Regenerate.
	* aarch64-dis.c (aarch64_decode_variant_using_iclass): Use new
	sve_size_13 icode to account for variant behaviour of
	pmull{t,b}.
	* aarch64-tbl.h (OP_SVE_VVV_HD_BS): Add new qualifier.
	(OP_SVE_VVV_Q_D): Add new qualifier.
	(OP_SVE_VVV_QHD_DBS): Remove now unused qualifier.
	(struct aarch64_opcode): Split pmull{t,b} into those requiring
	AES and those not.