x86: Encode AVX256/AVX512 vpsub[bwdq] with VEX128/EVEX128

x86: Encode AVX256/AVX512 vpsub[bwdq] with VEX128/EVEX128

When 2 source registers are identical, AVX256 and AVX512 vpsub[bwdq]
instructions can be encoded with VEX128 or EVEX128 encodings.

gas/

	* config/tc-i386.c (optimize_encoding): Optimize AVX256 and
	AVX512 vpsub[bwdq] instructions.
	* testsuite/gas/i386/optimize-1.s: Add tests for AVX256 and
	AVX512 vpsub[bwdq] instructions.
	* testsuite/gas/i386/x86-64-optimize-2.s: Likewise.
	* testsuite/gas/i386/optimize-1.d: Updated.
	* testsuite/gas/i386/x86-64-optimize-2.d: Likewise.

opcodes/

	* * i386-opc.tbl: Add "Optimize" to AVX256 and AVX512
	vpsub[bwdq] instructions.
	* i386-tbl.h: Regenerated.