[MLIR][NFC] loop transforms/analyis utils cleanup / modernize

Modernize/cleanup code in loop transforms utils - a lot of this code was
written prior to the currently available IR support / code style. This
patch also does some variable renames including inst -> op, comment
updates, turns getCleanupLoopLowerBound into a local function.

Differential Revision: https://reviews.llvm.org/D77175