target/mips: Remove JR opcode unused arguments

JR opcode (Jump Register) only takes 1 argument, $rs.
JALR (Jump And Link Register) takes 3: $rs, $rd and $hint.

Commit 6af0bf9c7c added their processing into decode_opc() as:

    case 0x08 ... 0x09: /* Jumps */
        gen_compute_branch(ctx, op1 | EXT_SPECIAL, rs, rd, sa);

having both opcodes handled in the same function: gen_compute_branch.

Per JR encoding, both $rd and $hint ('sa') are decoded as zero.

Later this code got extracted to decode_opc_special(),
commit 7a387fffce used definitions instead of magic values:

    case OPC_JR ... OPC_JALR:
        gen_compute_branch(ctx, op1, rs, rd, sa);

Finally commit 0aefa33318 moved OPC_JR out of decode_opc_special,
to a new 'decode_opc_special_legacy' function:

  @@ -15851,6 +15851,9 @@ static void decode_opc_special_legacy(CPUMIPSState *env, DisasContext *ctx)
  +    case OPC_JR:
  +        gen_compute_branch(ctx, op1, 4, rs, rd, sa);
  +        break;

  @@ -15933,7 +15936,7 @@ static void decode_opc_special(CPUMIPSState *env, DisasContext *ctx)
  -    case OPC_JR ... OPC_JALR:
  +    case OPC_JALR:
           gen_compute_branch(ctx, op1, 4, rs, rd, sa);
           break;

Since JR is now handled individually, it is pointless to decode
and pass it unused arguments. Replace them by simple zero value
to avoid confusion with this opcode.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210730225507.2642827-1-f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

target/mips/tcg/translate.c

@@ -14203,7 +14203,7 @@ static void decode_opc_special_legacy(CPUMIPSState *env, DisasContext *ctx)
         break;
 #endif
     case OPC_JR:
-        gen_compute_branch(ctx, op1, 4, rs, rd, sa, 4);
+        gen_compute_branch(ctx, op1, 4, rs, 0, 0, 4);
         break;
     case OPC_SPIM:
 #ifdef MIPS_STRICT_STANDARD