[mlir] Update how scalable indices are printed

This patch makes sure that scalable indices (that would normally
represent scalable tile or vector sizes) are printed correctly, i.e.
with additional square brackets:
```
%1, %loop = transform.structured.tile %0 [2, 8, [4]]
```

This change complements https://reviews.llvm.org/D150944 and is a part
of a larger effort to enable scalable vectorisation in Linalg. See this
RFC for more context:
  * https://discourse.llvm.org/t/rfc-scalable-vectorisation-in-linalg/

Differential Revision: https://reviews.llvm.org/D151978