target-i386: eliminate dead code and hoist common code out of "if"

ist != 0 is checked in the first "if", so it cannot be true in
the "else if" part.  While at it, simplify the code and move
the ESP alignment out of the conditionals.

Reported by Coverity.

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