[mlir] support conversion of parallel reduction loops to std

Recently introduced support for converting sequential reduction loops to
CFG of basic blocks in the Standard dialect makes it possible to perform
a staged conversion of parallel reduction loops into a similar CFG by
using sequential loops as an intermediate step. This is already the case
for parallel loops without reduction, so extend the pattern to support
an additional use case.

Differential Revision: https://reviews.llvm.org/D75599