Retain the name of the new internal global that's been shrunk.

It's possible (e.g. after an LTO build) that an internal global may be used for
debugging purposes. If that's the case appending a '.b' to it makes it hard to
find that variable. Steal the name from the old GV before deleting it so that
they can find that variable again.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175104 91177308-0d34-0410-b5e6-96231b3b80d8