target/i386: Fix decode of cr8

A recent cleanup did not recognize that there are two ways
to encode cr8: one via the LOCK and the other via REX.

Fixes: 7eff2e7c
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/380
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210602035511.96834-1-richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target/i386/tcg/translate.c

@@ -8091,6 +8091,7 @@ static target_ulong disas_insn(DisasContext *s, CPUState *cpu)
         case 2:
         case 3:
         case 4:
+        case 8:
             break;
         default:
             goto unknown_op;