target/i386: simplify handling of conforming code segments on interrupt

Move the handling of conforming code segments before the handling
of stack switch.

Because dpl == cpl after the new "if", it's now unnecessary to check
the C bit when testing dpl < cpl.  Furthermore, dpl > cpl is checked
slightly above the modified code, so the final "else" is unreachable
and we can remove it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
