[LoopDeletion] Handle users in unreachable block

This is a fix for PR35884.

When we want to delete dead loop we must clean uses in unreachable blocks
otherwise we'll get an assert during deletion of instructions from the loop.

Reviewers: anna, davide
Reviewed By: anna
Subscribers: llvm-commits, lebedev.ri
Differential Revision: https://reviews.llvm.org/D41943

llvm-svn: 322357