[mlir][Linalg] Avoid unnecessary propagating producer result to fused op result.

Elementwise op fusion conserves the result of the producer in the
fused op, relying on later clean up patterns to drop unused results of
the fused op. Instead, if the producer result has no other use apart
from the consumer op, avoid making the producer result available in
the fused node. This saves some unnecessary IR manipulations.

Differential Revision: https://reviews.llvm.org/D138096