[mlir] Add custom directive hooks for printing mixed integer or value operands.

Add printer and parser hooks for a custom directive that allows
parsing and printing of idioms that can represent a list of values
each of which is either an integer or an SSA value. For example in

`subview %source[%offset_0, 1] [4, %size_1] [%stride_0, 3]`

each of the list (which represents offset, size and strides) is a mix
of either statically know integer values or dynamically computed SSA
values. Since this is used in many places adding a custom directive to
parse/print this idiom allows using assembly format on operations
which use this idiom.

Differential Revision: https://reviews.llvm.org/D95773