Enhance instcombine to use the preferred field of
GetOrEnforceKnownAlignment in more cases, so that regular IR operations are
optimized in the same way that the intrinsics currently are.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@64623 91177308-0d34-0410-b5e6-96231b3b80d8