target/i386: do not decode string source/destination into decode->mem

target/i386: do not decode string source/destination into decode->mem

decode->mem is only used if one operand has has_ea == true.  String
operations will not use decode->mem and will load A0 on their own, because
they are the only case of two memory operands in a single instruction.

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