If we have a VLA that has a "use" in a metadata node that's then used

If we have a VLA that has a "use" in a metadata node that's then used
here but it has no other uses, then we have a problem. E.g.,

  int foo (const int *x) {
    char a[*x];
    return 0;
  }

If we assign 'a' a vreg and fast isel later on has to use the selection
DAG isel, it will want to copy the value to the vreg. However, there are
no uses, which goes counter to what selection DAG isel expects.
<rdar://problem/11134152>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@153705 91177308-0d34-0410-b5e6-96231b3b80d8