opcodes: gas: x86: define and use Rex2 as attribute not constraint

opcodes: gas: x86: define and use Rex2 as attribute not constraint

Rex2 is currently an operand constraint.  For the upcoming SCFI
implementation in GAS, we need to identify operations which implicitly
update the stack pointer.  An operand constraint enumerator for implicit
stack op seems more appropriate than an attribute.  However, two opcodes
currently necessitate both Rex2 and an implicit stack op marker; this
prompts revisiting the current representations a bit.

Make Rex2 a standalone attribute, so that later a new operand constraint
may be added for IMPLICIT_STACK_OP.

ChangeLog:
	* gas/config/tc-i386.c (is_apx_rex2_encoding): Update the check.
	* opcodes/i386-gen.c: Add a new BITFIELD for Rex2.
	* opcodes/i386-opc.h (REX2_REQUIRED): Remove.
	* opcodes/i386-opc.tbl: Remove Rex2 operand constraint.
	* opcodes/i386-tbl.h: Regenerated.