[DAGCombiner] Slightly simplify some code by using APInt::isMask() and countTrailingO...

[DAGCombiner] Slightly simplify some code by using APInt::isMask() and countTrailingOnes instead of getting active bits and checking if all the bits below that make a mask.

At least for the 64-bit and less case, we should be able to determine if we even have a mask without counting any bits. This also removes the need to explicitly check for 0 active bits, isMask will return false for 0.

llvm-svn: 313908