They can be simplified to reshape ops if outer_dims_perm is an identity
permutation. The revision adds a `isIdentityPermutation` method to
IndexingUtils.
A tensor.pack op can be rewritten to a tensor.expand_shape op if the
packing only happens on inner most dimension.
This also formats the lit checks better.
The revision moves pack/unpack related patterns to
PackAndUnpackPatterns.cpp. This follows the convention like other tensor
ops.
It also renames `populateSimplifyTensorPack` to
`populateSimplifyPackAndUnpackPatterns` and adds a TODO item for
tensor.unpack op.
