[SystemZ] Implement orderFrameObjects().

By reordering the objects on the stack frame after looking at the users, a
better utilization of displacement operands will result. This means less
needed Load Address instructions for the accessing of these objects.

This is important for very large functions where otherwise small changes
could cause a lot more/less accesses go out of range.

Note: this is not yet enabled for SystemZXPLINKFrameLowering, but should be.

Review: Ulrich Weigand

Differential Revision: https://reviews.llvm.org/D115690