Revert "[InstCombine] canonicalize trunc + insert as bitcast + shuffle, part 1 (2nd try)"

This reverts commit e71b81ca.

As discussed in the planned follow-on to this patch (D138874),
this and the subsequent patches in this set can cause trouble for
the backend, and there's probably no quick fix. We may even
want to canonicalize in the opposite direction (towards insertelt).