[mlir][Linalg] Make Elementwise op fusion return a map from existing values to...

[mlir][Linalg] Make Elementwise op fusion return a map from existing values to values in the fused op.

This replacement can be used to eliminate all uses of the
producer/consumer for case where producer/consumer has other uses
outside of the producer/consumer pair. This makes the
producer/consumer dead.

Add test and minor fixup to the test harness.

Reviewed By: hanchung

Differential Revision: https://reviews.llvm.org/D142848