Don't diddle VRSAVE if no registers need to be added/removed from it. This

Don't diddle VRSAVE if no registers need to be added/removed from it.  This
allows us to codegen functions as:

_test_rol:
        vspltisw v2, -12
        vrlw v2, v2, v2
        blr

instead of:

_test_rol:
        mfvrsave r2, 256
        mr r3, r2
        mtvrsave r3
        vspltisw v2, -12
        vrlw v2, v2, v2
        mtvrsave r2
        blr

Testcase here: CodeGen/PowerPC/vec_vrsave.ll

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@27777 91177308-0d34-0410-b5e6-96231b3b80d8