[mlir][linalg] Lower linalg to affine loops

Reviewers: nicolasvasilache

Reviewed By: nicolasvasilache

Subscribers: mgester, lucyrfox, merge_guards_bot, AlexEichenberger, mravishankar, ftynse, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, arpith-jacob, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72094