[AggressiveInstCombine] Add arithmetic shift right instr to `TruncInstCombine` DAG

Add `ashr` instruction to the DAG post-dominated by `trunc`, allowing
`TruncInstCombine` to reduce bitwidth of expressions containing
these instructions.

We should be shifting by less than the target bitwidth.
Also it is sufficient to require that all truncated bits
of the value-to-be-shifted are sign bits (all zeros or ones) and
one sign bit is left untruncated: https://alive2.llvm.org/ce/z/Ajo2__

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

Differential Revision: https://reviews.llvm.org/D108355