[DAGCombiner] visitShiftByConstant(): drop bogus signbit check

Summary:
That check claims that the transform is illegal otherwise.
That isn't true:
1. For `ISD::ADD`, we only process `ISD::SHL` outer shift => sign bit does not matter
   https://rise4fun.com/Alive/K4A
2. For `ISD::AND`, there is no restriction on constants:
   https://rise4fun.com/Alive/Wy3
3. For `ISD::OR`, there is no restriction on constants:
   https://rise4fun.com/Alive/GOH
3. For `ISD::XOR`, there is no restriction on constants:
   https://rise4fun.com/Alive/ml6

So, why is it there then?

This changes the testcase that was touched by @spatel in rL347478,
but i'm not sure that test tests anything particular?

Reviewers: RKSimon, spatel, craig.topper, jojo, rengolin

Reviewed By: spatel

Subscribers: javed.absar, llvm-commits, spatel

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61918

llvm-svn: 361044