Handle (shl (anyext (shr ...))) in SimpilfyDemandedBits

This is really an extension of the current (shl (shr ...)) -> shl optimization.
The main difference is that certain upper bits must also not be demanded.

The motivating examples are the first two in the testcase, which occur
in llvmpipe output.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192783 91177308-0d34-0410-b5e6-96231b3b80d8