aarch64: Reject no-Xt SYS aliases when Rt != 31 in disassembly

For SYS-encoding aliases that do not take Xt (for example TLBI ALLE1,
PLBI ALLE1), disassembly should not print the alias form when Rt != 31.
In that case, disassemble as the generic sys form instead.

Add a focused gas test that checks:
- Rt=31 still disassembles to aliases for no-Xt forms.
- Rt!=31 disassembles to sys ... , xN for those no-Xt forms.
- A valid Xt alias (TLBI VALE1) continues to disassemble as the alias.

opcodes/ChangeLog:

	* aarch64-dis.c (aarch64_ext_regrt_sysins): Mark present as
	whether Rt is non-default or the SYS op accepts Xt, so aliases
	that do not take Xt are rejected when Rt != 31.

gas/ChangeLog:

	* testsuite/gas/aarch64/sys-rt-alias.s: New test.
	* testsuite/gas/aarch64/sys-rt-alias.d: New test.

1 files changed, 3 insertions, 6 deletions

diff --git a/opcodes/aarch64-dis.c b/opcodes/aarch64-dis.c
index f09307847e6..8544ce4b6d4 100644
--- a/opcodes/aarch64-dis.c
+++ b/opcodes/aarch64-dis.c
@@ -331,12 +331,9 @@ aarch64_ext_regrt_sysins (const aarch64_operand *self, aarch64_opnd_info *info,
      help the disassembler determine whether this operand is optional or
      not.  */
 
-  if (aarch64_sys_ins_reg_tlbid_xt (inst->operands[0].sysins_op)
-      && info->reg.regno != 31)
-    info->present = true;
-  else
-    info->present = aarch64_sys_ins_reg_has_xt (inst->operands[0].sysins_op);
-
+  info->present
+    = (info->reg.regno != 31
+       || aarch64_sys_ins_reg_has_xt (inst->operands[0].sysins_op));
   return true;
 }