[mlir][Linalg] Fix linalg on tensor fusion

- Drop unnecessary occurrences of rewriter.eraseOp: dead linalg ops on tensors should be cleaned up by DCE.
- reimplement the part of Linalg on fusion that constructs the body and block arguments: the previous implementation had too much magic. Instead this spells out all cases explicitly and asserts / introduces TODOs for incorrect cases.

As a consequence, we can use the default traversal order for this pattern.

Differential Revision: https://reviews.llvm.org/D99070