As per title.
There are very few scalar tests - llvm\test\CodeGen\X86\known-bits.ll is almost empty. The llvm\test\CodeGen\X86\known-bits-vector.ll has more examples - scalarizing one of those should be trivial.
Add test case for add and adc.
The test case for adc is a bit complex, but it was necessary to put the adc in a context where its surrounding cannot be optimized away. That means computing a result using the carry, and returning it somehow, while making sure the result of the adc itself can be optimized away.