[DAG] visitSCALAR_TO_VECTOR - don't fold...

[DAG] visitSCALAR_TO_VECTOR - don't fold scalar_to_vector(bin(extract(x),extract(y)) -> bin(x,y) if extracts have other uses

Fixes #78897 - although the test case still has a number of poor codegen issues (in particular for i686 triples) that will need addressing (combining the nodes in topological order should help).