[mlir][Linalg] Add pass to remove unit-extent dims from tensor

operands of Generic ops.

Unit-extent dimensions are typically used for achieving broadcasting
behavior. The pattern added (along with canonicalization patterns
added previously) removes the use of unit-extent dimensions, and
instead uses a more canonical representation of the computation.  This
new pattern is not added as a canonicalization for now since it
entails adding additional reshape operations. A pass is added to
exercise these patterns, along with an API entry to populate a
patterns list with these patterns.

Differential Revision: https://reviews.llvm.org/D79766