Revert "Resurrect r191017 " GVN proceeds in the presence of dead code" plus a fix to PR17307 & 17308."

This causes PR17852.

This reverts commit d93e8a06b2ca09ab18f390cd514b7443e2e571f7.

Conflicts:
	test/Transforms/GVN/cond_br2.ll

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194348 91177308-0d34-0410-b5e6-96231b3b80d8