[MLIR][NFC] update/clean up affine PDT, related utils, its test case

- rename vars that had inst suffixes (due to ops earlier being
  known as insts); other renames for better readability
- drop unnecessary matches in test cases
- iterate without block terminator
- comment/doc updates
- instBodySkew -> affineForOpBodySkew

Signed-off-by: Uday Bondhugula <uday@polymagelabs.com>

Differential Revision: https://reviews.llvm.org/D76214