[mlir][linalg] Restrict distribution to parallel dims

According to the API contract, LinalgLoopDistributionOptions
expects to work on parallel iterators. When getting processor
information, only loop ranges for parallel dimensions should
be fed in. But right now after generating scf.for loop nests,
we feed in *all* loops, including the ones materialized for
reduction iterators. This can cause unexpected distribution
of reduction dimensions. This commit fixes it.

Reviewed By: mravishankar

Differential Revision: https://reviews.llvm.org/D102079