Copied directly from the IR version.
Most of the testcases I've added for this are somewhat problematic
because they really end up testing the yet to be implemented version
LG as a copy-paste from lib/Analysis/ValueTracking.cpp ComputeNumSignBitsImpl().
1 is a magic 'i-dont-know', right? I wonder why it's not -1,
Not sure if we need it for this patch, but I remember that last time I did something with computeNumSignBits in SelectionDAG I was informed that it is possible to add unittests in llvm/unittests/CodeGen/AArch64SelectionDAGTest.cpp. In case you for example want to make more detailed tests.