[BPI][NFC] Unify handling of normal and SCC based loops

This is one more NFC part extracted from D79485. Normal and SCC based loops have very different representation and have to be handled separatly each time we deal with loops. D79485 is going to introduce much more extensive use of loops what will be problematic with out this change.

Reviewed By: davidxl

Differential Revision: https://reviews.llvm.org/D84838