[SystemZ] Replace subreg_r with subreg_h

[SystemZ] Replace subreg_r with subreg_h

Change
  subreg_r32  -> subreg_h32
  subreg_r64  -> subreg_h64
  subreg_hr32 -> subreg_hh32

The subregisters subreg_r32 and subreg_r64 were added to emphasize the
fact that modifying these subregisters may clobber the entire register.
This is not necessarily the case for subreg_h32, et al.

However, the ability to compose subreg_h64 with subreg_r32, and with
subreg_h32 and subreg_l32 at the same time makes the compositions be
treated as non-overlapping (leading to problems when tracking subreg
liveness). See D50468 for more details.

Differential Revision: https://reviews.llvm.org/D50725

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339778 91177308-0d34-0410-b5e6-96231b3b80d8