[SystemZ] Extend 32-bit RISBG optimizations to high words

This involves using RISB[LH]G, whereas the equivalent z10 optimization
uses RISBG.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191770 91177308-0d34-0410-b5e6-96231b3b80d8