[SCEV] Remove computeLoadConstantCompareExitLimit() (NFCI)

The functionality of this method is already covered by
computeExitCountExhaustively() in a more general fashion. It was
added at a time when exhaustive exit count calculation did not
support constant folding loads yet. I double checked that dropping
this code causes no binary changes in test-suite.

Differential Revision: https://reviews.llvm.org/D112343