[IVDescriptors] Add assert to isInductionPhi to check for invalid Phis

Phis that are present inside loop headers can only be Induction Phis
legally. This patch adds an assertion to isInductionPhi which checks for
the said legality and it also updates the docs of the said function to
reflect the given legality.

Differential Revision: https://reviews.llvm.org/D149041