[DAGCombiner] Make the isTruncateOf call from visitZERO_EXTEND work for vectors. Remove FIXME.

I'm having trouble creating a test case for the ISD::TRUNCATE part of this that shows any codegen differences. But I was able to test the setcc path which is what the test changes here cover.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@345908 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed