[SystemZ] Extend RISBG optimization

The handling of ANY_EXTEND and ZERO_EXTEND was too strict.  In this context
we can treat ZERO_EXTEND in much the same way as an AND and then also handle
outermost ZERO_EXTENDs.

I couldn't find a test that benefited from the ANY_EXTEND change, but it's
more obvious to write it this way once SIGN_EXTEND and ZERO_EXTEND are
handled differently.

llvm-svn: 197802