target/i386: remove duplicate prefix decoding

target/i386: remove duplicate prefix decoding

Now that a bulk of opcodes go through the new decoder, it is sensible
to do some cleanup.  Go immediately through disas_insn_new and only jump
back after parsing the prefixes.

disas_insn() now only contains the three sigsetjmp cases, and they
are more easily managed if they are inlined into i386_tr_translate_insn.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>