[AggressiveInstCombine] Add `phi` nodes support to `TruncInstCombine`

Expand `TruncInstCombine` to handle loops by adding `phi` nodes
to expression graph.

Reviewed by: RKSimon, lebedev.ri

(recommit of fixed f84d732f, reverted by 8ad6d5e4 after sanitizer breakage)

Differential Revision: https://reviews.llvm.org/D109817