[SCEV][NFC-mostly] Remove constant handling in TripMultiple computation

After landing more precise trip multiples in
https://reviews.llvm.org/D149529, the SCEV multiple computation handles
constants, so there is no longer any need for special constant handling
in getSmallConstantTripMultiple.

This patch can improve the multiple of a non-constant SCEV that is huge
(>=2**32). This is very rare in practice.

Differential Revision: https://reviews.llvm.org/D150541