[Attributor] Set up a dedicated simplification call back map for `GlobalVariable`

Currently we don't check call backs for global variable simplification.
What's more, the only way that we can register a simplification call back for
global variable is through its initializer (essentially a `Constant *`). It might
not correspond to the right global variable. In this patch, we set up a dedicated
simplification map for `GlobalVariable`.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D144749