Use-and-clobber insns and some ancient combine.c code

[Richard Kenner]

     When a @code{clobber} expression for a register appears inside a
     @code{parallel} with other side effects, the register allocator
     guarantees that the register is unoccupied both before and after that
     insn.  However, the reload phase may allocate a register used for one of
     the inputs unless the @samp{&} constraint is specified for the selected
     alternative (@pxref{Modifiers}).

    These two sentences appear to directly contradict each other.

No, not at all.  The first talks about the register allocator (which ignores
a '&' constraint, and the latter talks abour reload (which doesn't ignore it).