[MLIR][OpenMP] Add omp.fuse operation (#168898)

This patch is a follow-up from #161213 and adds the omp.fuse loop
transformation for the OpenMP dialect. Used for lowering a `!$omp fuse`
in Flang.

Added Lowering and end2end tests.

GitOrigin-RevId: f560e4cfb1f9d3c71d7617f6e54f54eeba9f3239