DAG: Handle half spanning extract_subvector in type legalization (#154101)
Previously it would just assert if the extract needed elements from
both halves. Extract the individual elements from both halves and
create a new vector, as the simplest implementation. This could
try to do better and create a partial extract or shuffle (or
maybe that's best left for the combiner to figure out later).
Fixes secondary issue noticed as part of #153808