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.

gas/config/tc-i386.c

@@ -3955,7 +3955,7 @@ static INLINE bool
 is_apx_rex2_encoding (void)
 {
   return i.rex2 || i.rex2_encoding
-	|| i.tm.opcode_modifier.operandconstraint == REX2_REQUIRED;
+	|| i.tm.opcode_modifier.rex2;
 }
 
 static unsigned int

opcodes/i386-gen.c

@@ -490,6 +490,7 @@ static bitfield opcode_modifiers[] =
   BITFIELD (ISA64),
   BITFIELD (NoEgpr),
   BITFIELD (NF),
+  BITFIELD (Rex2),
 };
 
 #define CLASS(n) #n, n

opcodes/i386-opc.h

@@ -579,8 +579,6 @@ enum
   /* Instrucion requires that destination must be distinct from source
      registers.  */
 #define DISTINCT_DEST 9
-  /* Instrucion requires REX2 prefix.  */
-#define REX2_REQUIRED 10
   OperandConstraint,
   /* instruction ignores operand size prefix and in Intel mode ignores
      mnemonic size suffix check.  */
@@ -750,6 +748,9 @@ enum
   /* No CSPAZO flags update indication.  */
   NF,
 
+  /* Instrucion requires REX2 prefix.  */
+  Rex2,
+
   /* The last bitfield in i386_opcode_modifier.  */
   Opcode_Modifier_Num
 };
@@ -796,6 +797,7 @@ typedef struct i386_opcode_modifier
   unsigned int isa64:2;
   unsigned int noegpr:1;
   unsigned int nf:1;
+  unsigned int rex2:1;
 } i386_opcode_modifier;
 
 /* Operand classes.  */

opcodes/i386-opc.tbl

@@ -85,7 +85,6 @@
 #define RegKludge         OperandConstraint=REG_KLUDGE
 #define SwapSources       OperandConstraint=SWAP_SOURCES
 #define Ugh               OperandConstraint=UGH
-#define Rex2              OperandConstraint=REX2_REQUIRED
 
 #define ATTSyntax         Dialect=ATT_SYNTAX
 #define ATTMnemonic       Dialect=ATT_MNEMONIC