[ConstantFold] Handle identity folds at top of ConstantFoldBinaryInst

Currently we miss folds with undef and identity values for binary ops
that do not fold to undef in general.

We can generalize the identity simplifications and do them before
checking for undef in particular.

Alive checks:
 * OR - https://rise4fun.com/Alive/8OsK
 * AND - https://rise4fun.com/Alive/e3tE

This will also allow us to remove some now redundant cases throughout
the function, but I would like to do this as follow-up. That should make
tracking down potential issues easier.

Reviewers: spatel, RKSimon, lebedev.ri

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D70169