The DAGCombiner folds the zext into complex load instructions. This patch
prevents this optimization on vectors since none of the supported targets
knows how to perform load+vector_zext in one instruction.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@126080 91177308-0d34-0410-b5e6-96231b3b80d8
