[XLA:GPU] Add support for destructuring collapsing reshapes in symbolic tiles.

[copybara-service]

[XLA:GPU] Add support for destructuring collapsing reshapes in symbolic tiles.

Take a reshape [6,8] reshape([48]). The indexing map going through this
reshape from output to input would be
    (d0, d1) -> (d0 * 8 + d1).
Previously to this change, there was no support for extracting sizes and
strides from a multivariate expression.

Deriving a size_map for the indexing map above is as simple as creating a map
    (s0, s1) -> (s0 * s1),
which is easy enough. However, deriving a stride_map for the expression is
complicated. Conceptually, the stride of the composite expression should
correspond to the stride of the minormost dimension involved in the reshape
along which we capture more than a single element.

This holds because there are restrictions on what a valid tiling can be when
going through a collapse. Let s be an n-dimensional shape that is fully
collapsed. In order to be propagated successfully through the collapse, the
pattern of the tiling of s has to look like
    (1*, partial_dim?, full_dims*, 1*)
where full_dims are dimensions that are captured completely, and
partial_dim is a dimension that can be captured with an arbitrary tile.
This restriction is necessary to ensure that the gap between two elements
captured in the expression is always the same (i.e., the set of elements can
be described using a single stride and is thus a tile).

Based on the above, algorithm for extracting the stride could therefore be
represented as a series of nested if statements
    if size0 != 1 then stride0 else (if size1 != 1 then stride1 else ...)
where {size,stride}i corresponds to the i-th major {size,stride}.

We implement a utility function that allows us to generate if statements as
affine expressions.