Fix two bugpoint bugs:

Fix two bugpoint bugs:

1) opt is not usually in the same path as the target program. Even for
the bugpoint as a standalone app, it should be more portable to search
in PATH, isn't it?
2) bugpoint driver accounts opt plugins, but does not list them in the
final output command.

Patch by Dmitry Mikushin!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@153066 91177308-0d34-0410-b5e6-96231b3b80d8