[DAGCombiner] Remove duplicate code from visitZERO_EXTEND

This exact block of code exists right below.

Differential Revision: https://reviews.llvm.org/D38122

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313891 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed