Revert "GlobalDCE: Delete available_externally initializers if it allows...

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.

llvm-svn: 212550