[MLIR] Propagate unpack through element-wise ops

Introduce `pushDownUnPackOpThroughElemGenericOp` to propagate producer
unpack operation through an element-wise linalg.generic operation. This
pattern complements `BubbleUpPackOpThroughElemGenericOp`. The general
idea is to bubble up tensor.pack as much as possible while pushing down
tensor.unpack as much as possible, and canonicalize away symmetrical
tensor.pack and tensor.unpack operations.

Currently, `pushDownUnPackOpThroughElemGenericOp` expects a single
tensor.unpack operation as the producer of one of the linalg.generic's
operands.

Reviewed By: hanchung

Differential Revision: https://reviews.llvm.org/D142523