[mlir] parallel loop canonicalization

Summary:
The patch introduces a canonicalization pattern for parallel loops. The pattern removes single-iteration loop dimensions if the loop bounds and steps are constants.

Reviewers: herhut, ftynse

Reviewed By: herhut

Subscribers: mehdi_amini, rriddle, jpienaar, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, stephenneuendorffer, Joonsoo, grosul1, Kayjukh, jurahul, msifontes

Tags: #mlir

Differential Revision: https://reviews.llvm.org/D82191