Revert "GlobalDCE: Delete available_externally initializers if it allows removing the value the initializer is referring to."

This reverts commit 5b55a47e94e28fbb56d0cd5d72c3db9105c15b4c.

A test case was found to crash after this was applied.  I'll file a bug to track fixing this with the test case needed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212550 91177308-0d34-0410-b5e6-96231b3b80d8