ValueTracking: Figure out more bits when looking at add/sub

Given something like X01XX + X01XX, we know that the result must look
like X1XXX.

Adapted from a patch by Richard Smith, test-case written by me.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216250 91177308-0d34-0410-b5e6-96231b3b80d8