[MLIR] Fix incorrect removal of source loop in loop fusion

This patch fixes a bug in loop fusion pass where the source loop was removed
even when the fused loop did not cover all iterations of the source loop.

This was because the fast hueristic check for checking if source loop and
fused loop have same iterations did not take into account steps in loop.

Reviewed By: dcaballe, bondhugula

Differential Revision: https://reviews.llvm.org/D114164