[AggressiveInstCombine] Add `udiv` and `urem` instrs to TruncInstCombine DAG

Add `udiv` and `urem` instructions to the DAG post-dominated by `trunc`,
allowing TruncInstCombine to reduce bitwidth of expressions containing these
instructions. It is sufficient to require that all truncated bits of both
operands are zeros: https://alive2.llvm.org/ce/z/yiithn
(`urem` case is identical).

Differential Revision: https://reviews.llvm.org/D109515