[SystemZ] Rename 32-bit GPR registers

[SystemZ] Rename 32-bit GPR registers

I'm about to add support for high-word operations, so it seemed better
for the low-word registers to have names like R0L rather than R0W.
No behavioral change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191655 91177308-0d34-0410-b5e6-96231b3b80d8