Clone and restore the module being reduced in

Clone and restore the module being reduced in
ReduceMiscompilingFunctions::TestFuncs. This makes the test functional
(i.e., no side effects).

Before we would end up using dead functions if a pass decided to remove them
(inline for example) and we would also keep broken functions and conclude that
that a single function was enough to reproduce the bug.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@109387 91177308-0d34-0410-b5e6-96231b3b80d8