[MLIR][Linalg] Avoid generalizing `linalg.map`

We cannot trivially generalize `linalg.map`, as it does not use the
output as a region argument in the block, while `linalg.generic` expects
many region arguments as the input/output operands.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D153442