[SCCP] Don't check for UndefValue before calling markConstant()

[SCCP] Don't check for UndefValue before calling markConstant()

The value lattice explicitly represents undef, and markConstant()
internally checks for UndefValue and will create an undef rather
than constant lattice element in that case.

This is mostly a code simplification, it has little practical impact
because we usually get undef results from undef operands, and those
don't get processed.

Only leave the check behind for the CmpInst case, because it
currently goes through this incorrect code in the getCompare()
implementation: https://github.com/llvm/llvm-project/blob/f98697642cea761448dc0f84f750d3f5def8af6b/llvm/include/llvm/Analysis/ValueLattice.h#L456-L457

Differential Revision: https://reviews.llvm.org/D128330