bugpoint Enhancement.

Summary:
This patch adds two flags to `bugpoint`: "-replace-funcs-with-null" and "-disable-pass-list-reduction".

When "-replace-funcs-with-null" is specified, bugpoint will, instead of simply deleting function bodies, replace all uses of functions and then will delete functions completely from the test module, correctly handling aliasing and @llvm.used && @llvm.compiler.used. This part was conceived while trying to debug the PNaCl IR simplification passes, which don't allow undefined functions (ie no declarations).

With "-disable-pass-list-reduction", bugpoint won't try to reduce the set of passes causing the "crash". This is needed in cases where one is trying to debug an issue inside the PNaCl IR simplification passes which is causing an PNaCl ABI verification error, for example.

Reviewers: jfb

Reviewed By: jfb

Subscribers: jfb, llvm-commits

Differential Revision: http://reviews.llvm.org/D8555

llvm-svn: 235362