[SCEV] `createNodeForSelectOrPHIInstWithICmpInstCond()`: generalize eq handling

The current logic was: https://alive2.llvm.org/ce/z/j8muXk
but in reality the offset to the Y in the 'true' hand
does not need to exist: https://alive2.llvm.org/ce/z/MNQ7DZ
https://alive2.llvm.org/ce/z/S2pMQD

To catch that, instead of computing the Y's in both
hands and checking their equality, compute Y and C,
and check that C is 0 or 1.