[LoopUnswitch] Trivial simplification: remove trivial dead condition after unswitch

Non trivial loop unswitch can keep the dead condition instruction.
CL adds trivial dead code elimination for unused condition.

Reviewers: asbirlea, aqjune, fhahn, DaniilSuchkov, reames
Reviewed By: asbirlea
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D88014