[mlir] Move some linalg patterns around.

The bufferization patterns are moved to the .cpp file, which is
preferred in the codebase when it makes sense.

The LinalgToStandard patterns are kept a header because they are
expected to be used individually. However, they are moved to
LinalgToStandard.h which is the file corresponding to where they are
defined.

This also removes TensorCastOpConverter, which is handled by
populateStdBufferizePatterns now. Eventually, the constant op lowering
will be handled as well, but it there are currently holdups on moving
it (see https://reviews.llvm.org/D89916).

Differential Revision: https://reviews.llvm.org/D90254