Currently empty tensor elimination by constructing a SubsetExtractionOp
to match a SubsetInsertionOp at the end of a DPS chain will fail if any
operands required by the insertion op don't dominate the insertion point
for the extraction op.
This change improves the transformation by attempting to move all pure
producers of required operands to the insertion point of the extraction
op. In the process this improves a number of tests for empty tensor
elimination.
bb17dfa7d1