[Bugpoint redesign] Added Pass to Remove Global Variables

[Bugpoint redesign] Added Pass to Remove Global Variables

Summary:
This pass tries to remove Global Variables, as well as their derived uses. For example if a variable `@x` is used by `%call1` and `%call2`, both these uses and the definition of `@x` are deleted. Moreover if `%call1` or `%call2` are used elsewhere those uses are also deleted, and so on recursively.

I'm still uncertain if this pass should remove derived uses, I'm open to suggestions.

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64176

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@368918 91177308-0d34-0410-b5e6-96231b3b80d8