[mlir][Linalg] NFC: Move populatePatterns* method into linalg namespace.

The moved `populate` methods are only relevant to Linalg
operations. So they are better of in `linalg` namespace.  Also rename
`populateLinalgTensorOpsFusionPatterns` to
`populateElementwiseOpsFusionPatterns`. This makes the scope of these
patterns explicit and disambiguates it with fusion on tensors using
tile + fuse.

Differential Revision: https://reviews.llvm.org/D99819