[AggressiveInstCombine] Add `{insert/extract}element` to `TruncInstCombine` DAG

Alive2 for `{insert/extract}element`: https://alive2.llvm.org/ce/z/hwy_E-

Actually, no one file of test suite is touched by this change,
which means that is rare pattern not generated by frontend. But
it's worth being in place.

Differential Revision: https://reviews.llvm.org/D109236