[SystemZ] Add STOC and STOCG

These instructions are allowed to trap even if the condition is false,
so for now they are only used for "*ptr = (cond ? x : *ptr)"-style
constructs.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187111 91177308-0d34-0410-b5e6-96231b3b80d8