[DAG] Always use stack to promote bitcast when the source is vector (#151065)

The optimization introduced by #125637 tried to avoid using stacks to
promote bitcast with vector result type. However, it wouldn't be correct
if the input type is vector. This patch limits that optimizations to
only scalar to vector bitcasts.

GitOrigin-RevId: 7ebbbd885f759ef5b7ae50a1b07f810296c4967f
12 files changed