[JumpThreading] Clone noalias.scope.decl when threading blocks

When cloning instructions during jump threading, also clone and
adapt any declared scopes. This is primarily important when
threading loop exits, because we'll end up with two dominating
scope declarations in that case (at least after additional loop
rotation). This addresses a loose thread from
https://reviews.llvm.org/rG2556b413a7b8#975012.

Differential Revision: https://reviews.llvm.org/D97154

(cherry picked from commit 5e7e499b)