[DAGCombiner] Factor out distributeTruncateThroughAnd

Currently this code is duplicated across visitSHL, visitSRA and visitSRL.  The
plan is to add rotates as clients to this new function.

There is no functional change intended here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202908 91177308-0d34-0410-b5e6-96231b3b80d8