[bugpoint] Simplify the global initializer reducer, NFC
[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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@324649 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 0f1e6f8445e769f14004c819a74840e9e4c9471c)

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm