target/mips: Convert Rel6 Special2 opcode to decodetree

Special2 opcode have been removed from the Release 6.

Add a single decodetree entry for all the opcode class,
triggering Reserved Instruction if ever used.

Remove unreachable check_insn_opc_removed() call.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20201208203704.243704-7-f4bug@amsat.org>

target/mips/mips32r6.decode

@@ -15,3 +15,5 @@
 @lsa                ...... rs:5 rt:5 rd:5 ... sa:2 ......   &rtype
 
 LSA                 000000 ..... ..... ..... 000 .. 000101  @lsa
+
+REMOVED             011100 ----- ----- ----- ----- ------   # SPECIAL2

target/mips/rel6_translate.c

@@ -18,6 +18,13 @@
 #include "decode-mips32r6.c.inc"
 #include "decode-mips64r6.c.inc"
 
+bool trans_REMOVED(DisasContext *ctx, arg_REMOVED *a)
+{
+    gen_reserved_instruction(ctx);
+
+    return true;
+}
+
 static bool trans_LSA(DisasContext *ctx, arg_rtype *a)
 {
     return gen_lsa(ctx, a->rd, a->rt, a->rs, a->sa);

target/mips/translate.c

@@ -27136,8 +27136,6 @@ static void decode_opc_special2_legacy(CPUMIPSState *env, DisasContext *ctx)
     int rs, rt, rd;
     uint32_t op1;
 
-    check_insn_opc_removed(ctx, ISA_MIPS_R6);
-
     rs = (ctx->opcode >> 21) & 0x1f;
     rt = (ctx->opcode >> 16) & 0x1f;
     rd = (ctx->opcode >> 11) & 0x1f;