[ValueTracking] Clarify TypeSize comparisons

TypeSize comparisons using overloaded operators should be replaced by
the new isKnownXY comparators when the operands can be fixed-length or
scalable vectors.

In ValueTracking there are several uses of the overloaded operators in
`isKnownNonZero` and `ComputeMultiple`. In the former we already bail
out on scalable vectors since we currently have no way to represent
DemandedElts, and the latter is operating on scalar integers, so we can
assume fixed-size in both instances.

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D89387