[bugpoint] Add a named metadata (+their operands) reducer

[bugpoint] Add a named metadata (+their operands) reducer

Summary:
We frequently run bugpoint on a linked module that consists of all
modules we create while jitting the julia standard library. This module
has a very large number of compile units (10000+) in `llvm.dbg.cu`,
which didn't get reduced at all, requiring manual post processing.
This is an attempt to have bugpoint go through and attempt to reduce
the number of global named metadata nodes as well as their operands,
to cut down the number of roots for such metadata.

Reviewers: dexonsmith, reames, pete

Subscribers: pete, dexonsmith, reames, llvm-commits

Differential Revision: http://reviews.llvm.org/D14043

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@252247 91177308-0d34-0410-b5e6-96231b3b80d8