[SystemZ] Use upper words of GR64s for codegen

This just adds the basics necessary for allocating the upper words to
virtual registers (move, load and store).  The move support is parameterised
in a way that makes it easy to handle zero extensions, but the associated
zero-extend patterns are added by a later patch.

The easiest way of testing this seemed to be add a new "h" register
constraint for high words.  I don't expect the constraint to be useful
in real inline asms, but it should work, so I didn't try to hide it
behind an option.

llvm-svn: 191739