Extend AffineToGPU to support Linalg loops

Extend the utility that converts affine loop nests to support other types of
loops by abstracting away common behavior through templates.  This also
slightly simplifies the existing Affine to GPU conversion by always passing in
the loop step as an additional kernel argument even though it is a known
constant.  If it is used, it will be propagated into the loop body by the
existing canonicalization pattern and can be further constant-folded, otherwise
it will be dropped by canonicalization.

This prepares for the common loop abstraction that will be used for converting
to GPU kernels, which is conceptually close to Linalg loops, while maintaining
the existing conversion operational.

PiperOrigin-RevId: 257172216