[mlir] Small cleanups to func-bufferize/finalizing-bufferize

- Address TODO in scf-bufferize: the argument materialization issue is
  now fixed and the code is now in Transforms/Bufferize.cpp
- Tighten up finalizing-bufferize to avoid creating invalid IR when
  operand types potentially change
- Tidy up the testing of func-bufferize, and move appropriate tests
  to a new finalizing-bufferize.mlir
- The new stricter checking in finalizing-bufferize revealed that we
  needed a DimOp conversion pattern (found when integrating into npcomp).
  Previously, the converion infrastructure was blindly changing the
  operand type during finalization, which happened to work due to
  DimOp's tensor/memref polymorphism, but is generally not encouraged
  (the new pattern is the way to tell the conversion infrastructure that
  it is legal to change that type).