IR: Make metadata typeless in assembly

Now that `Metadata` is typeless, reflect that in the assembly.  These
are the matching assembly changes for the metadata/value split in
r223802.

  - Only use the `metadata` type when referencing metadata from a call
    intrinsic -- i.e., only when it's used as a `Value`.

  - Stop pretending that `ValueAsMetadata` is wrapped in an `MDNode`
    when referencing it from call intrinsics.

So, assembly like this:

    define @foo(i32 %v) {
      call void @llvm.foo(metadata !{i32 %v}, metadata !0)
      call void @llvm.foo(metadata !{i32 7}, metadata !0)
      call void @llvm.foo(metadata !1, metadata !0)
      call void @llvm.foo(metadata !3, metadata !0)
      call void @llvm.foo(metadata !{metadata !3}, metadata !0)
      ret void, !bar !2
    }
    !0 = metadata !{metadata !2}
    !1 = metadata !{i32* @global}
    !2 = metadata !{metadata !3}
    !3 = metadata !{}

turns into this:

    define @foo(i32 %v) {
      call void @llvm.foo(metadata i32 %v, metadata !0)
      call void @llvm.foo(metadata i32 7, metadata !0)
      call void @llvm.foo(metadata i32* @global, metadata !0)
      call void @llvm.foo(metadata !3, metadata !0)
      call void @llvm.foo(metadata !{!3}, metadata !0)
      ret void, !bar !2
    }
    !0 = !{!2}
    !1 = !{i32* @global}
    !2 = !{!3}
    !3 = !{}

I wrote an upgrade script that handled almost all of the tests in llvm
and many of the tests in cfe (even handling many `CHECK` lines).  I've
attached it (or will attach it in a moment if you're speedy) to PR21532
to help everyone update their out-of-tree testcases.

This is part of PR21532.

llvm-svn: 224257
diff --git a/llvm/test/Transforms/Inline/noalias.ll b/llvm/test/Transforms/Inline/noalias.ll
index 7a54d5d..76e4508 100644
--- a/llvm/test/Transforms/Inline/noalias.ll
+++ b/llvm/test/Transforms/Inline/noalias.ll
@@ -64,13 +64,13 @@
 
 attributes #0 = { nounwind uwtable }
 
-; CHECK: !0 = metadata !{metadata !1}
-; CHECK: !1 = metadata !{metadata !1, metadata !2, metadata !"hello: %a"}
-; CHECK: !2 = metadata !{metadata !2, metadata !"hello"}
-; CHECK: !3 = metadata !{metadata !4, metadata !6}
-; CHECK: !4 = metadata !{metadata !4, metadata !5, metadata !"hello2: %a"}
-; CHECK: !5 = metadata !{metadata !5, metadata !"hello2"}
-; CHECK: !6 = metadata !{metadata !6, metadata !5, metadata !"hello2: %b"}
-; CHECK: !7 = metadata !{metadata !4}
-; CHECK: !8 = metadata !{metadata !6}
+; CHECK: !0 = !{!1}
+; CHECK: !1 = !{!1, !2, !"hello: %a"}
+; CHECK: !2 = !{!2, !"hello"}
+; CHECK: !3 = !{!4, !6}
+; CHECK: !4 = !{!4, !5, !"hello2: %a"}
+; CHECK: !5 = !{!5, !"hello2"}
+; CHECK: !6 = !{!6, !5, !"hello2: %b"}
+; CHECK: !7 = !{!4}
+; CHECK: !8 = !{!6}