arc: Put DBNZ instruction to a separate class

arc: Put DBNZ instruction to a separate class

DBNZ instruction decrements its source register operand, and if
the result is non-zero it branches to the location defined by a signed
half-word displacement operand.

DBNZ instruction is in BRANCH class as other branch instrucitons
like B, Bcc, etc. However, DBNZ is the only branch instruction
that stores a branch offset in the second operand. Thus it must
be placed in a distinct class and treated differently.

For example, current logic of arc_insn_get_branch_target in GDB
assumes that a branch offset is always stored in the first operand
for BRANCH class and it's wrong for DBNZ.

include/ChangeLog:

2024-02-14  Yuriy Kolerov  <ykolerov@synopsys.com>

	* opcode/arc.h (enum insn_class_t): Add DBNZ class.

opcodes/ChangeLog:

2024-02-14  Yuriy Kolerov  <ykolerov@synopsys.com>

	* arc-tbl.h (dbnz): Use "DBNZ" class.
	* arc-dis.c (arc_opcode_to_insn_type): Handle "DBNZ" class.

gas/ChangeLog:

2024-02-14  Yuriy Kolerov  <ykolerov@synopsys.com>

	* config/tc-arc.c (is_br_jmp_insn_p): Add check against "DBNZ".