[PATCH 31/57][Arm][GAS] Add support for MVE instructions: vshrn[tb], vrshrn[tb],...

[PATCH 31/57][Arm][GAS] Add support for MVE instructions: vshrn[tb], vrshrn[tb], vqshrn[tb], vqshrun[tb], vqrshrn[tb] and vqrshrun[tb]

gas/ChangeLog:
2019-05-16  Andre Vieira  <andre.simoesdiasvieira@arm.com>

	* config/tc-arm.c (M_MNEM_vshrnt, M_MNEM_vshrnb, M_MNEM_vrshrnt,
	 M_MNEM_vqshrnt, M_MNEM_vqshrnb, M_MNEM_vqshrunt, M_MNEM_vqshrunb,
	 M_MNEM_vrshrnb, M_MNEM_vqrshrnt, M_MNEM_vqrshrnb, M_MNEM_vqrshrunt,
	 M_MNEM_vqrshrunb): New instruction encodings.
	(do_mve_vshrn): New encoding function.
	(insns): Add entries for MVE mnemonics.
	* testsuite/gas/arm/mve-vqrshrn-bad.d: New test.
	* testsuite/gas/arm/mve-vqrshrn-bad.l: New test.
	* testsuite/gas/arm/mve-vqrshrn-bad.s: New test.
	* testsuite/gas/arm/mve-vshrn-bad.d: New test.
	* testsuite/gas/arm/mve-vshrn-bad.l: New test.
	* testsuite/gas/arm/mve-vshrn-bad.s: New test.