[nfc] Refactor SlotIndex::getInstrDistance to better reflect actual functionality

[nfc] Refactor SlotIndex::getInstrDistance to better reflect actual functionality

This patch refactors SlotIndex::getInstrDistance to
SlotIndex::getApproxInstrDistance to better describe the actual
functionality of this function. This patch also adds in some additional
comments better documenting the assumptions that this function makes to
increase clarity.

Based on discussion on the LLVM Discourse:
https://discourse.llvm.org/t/odd-behavior-in-slotindex-getinstrdistance/64934/5

Reviewed By: mtrofin, foad

Differential Revision: https://reviews.llvm.org/D133386