[DAG] visitINSERT_VECTOR_ELT - use mergeEltWithShuffle to merge inserted...

[DAG] visitINSERT_VECTOR_ELT - use mergeEltWithShuffle to merge inserted vector element chain into base shuffle node

This allows us to merge insert_elt(insert_elt(shuffle(x,y),extract_elt(x,c1),c2),extract_elt(y,c3),c4) style insertion chains into a new shuffle node.

I had hoped to remove mergeInsertEltWithShuffle entirely, but that case doesn't have the one use limits so we would regress in a few other cases.

Fixes the vector-shuffle-combining.ll regressions in D127115