[AggressiveInstCombine] Add shift left instruction to `TruncInstCombine` DAG

Add `shl` instruction to the DAG post-dominated by `trunc`, allowing
TruncInstCombine to reduce bitwidth of expressions containing left shifts.

The only thing we need to check is that the target bitwidth must be wider
than the maximal shift amount: https://alive2.llvm.org/ce/z/AwArqu

Part of https://reviews.llvm.org/D107766

Differential Revision: https://reviews.llvm.org/D108091