[SystemZ] Extend the use of C(L)GFR

instcombine prefers to put extended operands first, so this patch
handles that case for C(L)GFR.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196579 91177308-0d34-0410-b5e6-96231b3b80d8
[SystemZ] Use LOAD AND TEST for comparisons with -0

...since it os equivalent to comparison with +0.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196580 91177308-0d34-0410-b5e6-96231b3b80d8