[mlir][SCF] Modernize `coalesceLoops` method to handle `scf.for` loops with iter_args (#87019)

As part of this extension this change also does some general cleanup

1) Make all the methods take `RewriterBase` as arguments instead of
   creating their own builders that tend to crash when used within
   pattern rewrites
2) Split `coalesePerfectlyNestedLoops` into two separate methods, one
   for `scf.for` and other for `affine.for`. The templatization didnt
   seem to be buying much there.

Also general clean up of tests.