[MLIR] Add loop coalesce utility for affine.for

Add loop coalesce utility for affine.for. This expects loops to have
been normalized a-priori. This works for both constant as well non
constant upper bounds having single/multiple result upper bound affine
map.

With contributions from Arnab Dutta and Uday Bondhugula.

Reviewed By: bondhugula, ayzhuang

Differential Revision: https://reviews.llvm.org/D108126