[SCEVAffinator] Fix assertion checking for constant divisor.

An assertion in visitSDivInstruction() checked whether the divisor is constant
by checking whether the argument is a ConstantInt. However, SCEVValidator allows
the divisor to be simplified to a constant by ScalarEvolution.

We synchronize the implementation of SCEVValidator and SCEVAffinator to both
accept simplified SCEV expressions.

llvm-svn: 275174