Add a pass that specializes parallel loops for easier unrolling and vectorization

This matches loops with a affine.min upper bound, limiting the trip
count to a constant, and rewrites them into two loops, one with constant
upper bound and one with variable upper bound. The assumption is that
the constant upper bound loop will be unrolled and vectorized, which is
preferable if this is the hot path.

Differential Revision: https://reviews.llvm.org/D75240