InstCombe: Infer nsw for multiplies

We already utilize this logic for reducing overflow intrinsics, it makes
sense to reuse it for normal multiplies as well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224847 91177308-0d34-0410-b5e6-96231b3b80d8
ValueTracking: Small cleanup in ComputeNumSignBits

Constant contains the isAllOnesValue and isNullValue predicates, not
ConstantInt.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224848 91177308-0d34-0410-b5e6-96231b3b80d8