[bugpoint] Simplify the global initializer reducer, NFC

Fix the comments, use early exits, use unique_ptr, and use ranged for
loops.

This is in preparation for a global *variable* reducer, which, with any
luck will help us clean up test cases.

Differential Revision: https://reviews.llvm.org/D43084

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@324649 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed