[mlir][Linalg] Generalize the definition of a Linalg contraction.

This revision defines a Linalg contraction in general terms:

  1. Has 2 input and 1 output shapes.
  2. Has at least one reduction dimension.
  3. Has only projected permutation indexing maps.
  4. its body computes `u5(u1(c) + u2(u3(a) * u4(b)))` on some field
    (AddOpType, MulOpType), where u1, u2, u3, u4 and u5 represent scalar unary
    operations that may change the type (e.g. for mixed-precision).

As a consequence, when vectorization of such an op occurs, the only special
behavior is that the (unique) MulOpType is vectorized into a
`vector.contract`. All other ops are handled in a generic fashion.

 In the future, we may wish to allow more input arguments and elementwise and
 constant operations that do not involve the reduction dimension(s).

A test is added to demonstrate the proper vectorization of matmul_i8_i8_i32.

Differential revision: https://reviews.llvm.org/D95939