[mlir][Linalg] Preserve discardable/user-defined attributes during generalization (#178599)
-- As observed in a [downstream
project](https://github.com/iree-org/iree/pull/23294#discussion_r2734982998)
: the named to generize linalg op conversion wasn't preserving
discardable attributes.
-- This commit aims to fix the same.
-- Only a single test case is added as the change applies to any named
linalg op's generalization.
Signed-off-by: Abhishek Varma <abhvarma@amd.com>