[mlir][Linalg] Canonicalize duplicate args.

I ran into this pattern when converting elementwise ops like
`addf %arg0, %arg : tensor<?xf32>` to linalg. Redundant arguments can
also easily arise from linalg-fusion-for-tensor-ops.

Also, fix some small bugs in the logic in
LinalgStructuredOpsInterface.td.

Differential Revision: https://reviews.llvm.org/D90812