[SystemZ] Try to fold shifts into TMxx

E.g. "SRL %r2, 2; TMLL %r2, 1" => "TMLL %r2, 4".


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190672 91177308-0d34-0410-b5e6-96231b3b80d8