[mlir][linalg] Audit the usage of casts in generic bodies for specialised op conversion

This [patch](https://github.com/llvm/llvm-project/pull/174757) fixes the issue of interpreting casts during `linalg.matmul` op variant specialisation from `linalg.generic` bodies (the original bug was filed here: https://github.com/llvm/llvm-project/issues/174517). Audit the presence of this issue in other specialisation cases and fix them in a potentially similar manner.

CC: @banach-space @adam-smnk

Start by reviewing the linked patch 174757 and original issue 174517, then inspect other linalg generic-body specialisation cases for the same cast interpretation problem. Done means affected cases are identified and fixed consistently with the matmul case, with validation for the affected conversions.

Written by the indexing model from the issue text.