[InstCombine] Move ptrtoaddr tests to InstSimplify (NFC)
All the existing tests test code either in ConstantFolding or
InstSimplify, so move them to use -passes=instsimplify instead of
-passes=instcombine. This makes sure we keep InstSimplify coverage
even if there are subsuming InstCombine folds.
This requires writing some of the constant folding tests in a
different way, as InstSimplify does not try to re-fold already
existing constant expressions.
