[SCCP] Pass the Solver by reference, copies are expensive ...

.. enough to cause LTO compile time to regress insanely.
Thanks *a lot* to Rafael for reporting the problem and testing
the fix!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@275468 91177308-0d34-0410-b5e6-96231b3b80d8