[SCEV] Make getRange smarter around selects

Have ScalarEvolution::getRange re-consider cases like "{C?A:B,+,C?P:Q}"
by factoring out "C" and computing RangeOf{A,+,P} union RangeOf({B,+,Q})
instead.

The latter can be easier to compute precisely in cases like
"{C?0:N,+,C?1:-1}" N is the backedge taken count of the loop; since in
such cases the latter form simplifies to [0,N+1) union [0,N+1).

llvm-svn: 262438