[mlir][sparse] Simplifying Merger::expContainsTensor

`expContainsTensor` used to call `expIsTensor` to short-circuit the recursive calls; however, the very first thing `expContainsTensor` does is to check `expIsTensor`, so the short-circuiting code just causes the function to check that condition redundantly.

Depends On D146684

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D146688