Revert "Reorder linalg.conv indexing_maps loop order"

This reverts commit 9b475258
and falls back to the original parallel-iterators-as-leading-
dimensions convention. We can control the loop order by first
converting the named op into linalg.generic and then performing
interchange.

Reviewed By: nicolasvasilache, asaadaldien

Differential Revision: https://reviews.llvm.org/D91796