Re-revert: Recover debug intrinsics when killing duplicated/empty blocks

This reverts commit 61b35e41.

This commit causes a timeout in chromium builds; likely to have a
similar cause to the previous timeout issue caused by this commit (see
6ded69f2 for more details). It is possible that there is no way to
fix this bug that will not cause this issue; further investigations as
to the efficiency of handling large amounts of debug info will be
necessary.