[SCEV] Slightly generalize getRangeViaFactoring

Building on the previous change, this generalizes
ScalarEvolution::getRangeViaFactoring to work with
{Ext(C?A:B)+k0,+,Ext(C?A:B)+k1} where Ext can be a zero extend, sign
extend or truncate operation, and k0 and k1 are constants.

llvm-svn: 262979