[mlir][Linalg] Miscalleneous enhancements to cover more fusion cases.

Adds support for
- Dropping unit dimension loops for indexed_generic ops.
- Folding consecutive folding (or expanding) reshapes when the result
  (or src) is a scalar.
- Fixes to indexed_generic -> generic fusion when zero-dim tensors are
  involved.

Differential Revision: https://reviews.llvm.org/D90118