[DAG] combineInsertEltToShuffle - split off mergeInsertEltWithShuffle fold. NFC.

combineInsertEltToShuffle was performing 2 very different folds in the same call, merging "(insert_vector_elt (vector_shuffle X, Y), (extract_vector_elt X, N), IdxC) --> (vector_shuffle X, Y)" and "(insert_vector_elt V, (bitcast X from vector type), IdxC) --> bitcast(shuffle (bitcast V), (extended X), Mask)"

The folds are currently still attempted in the same order as before (just as 2 seperate calls) so there should be no change in behaviour.

First step towards some adjustments to mergeInsertEltWithShuffle for D127115.