[mlir][Linalg] Move named op conversions out of canonicalizations.

These conversions are better suited to be applied at whole tensor
level. Applying these as canonicalizations end up triggering such
canonicalizations at all levels of the stack which might be
undesirable. For example some of the resulting code patterns wont
bufferize in-place and need additional stack buffers. Best is to be
more deliberate in when these canonicalizations apply.

Differential Revision: https://reviews.llvm.org/D115912