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/GlobalOpt/metadata.ll b/llvm/test/Transforms/GlobalOpt/metadata.ll
index f3740e1..fb60b66 100644
--- a/llvm/test/Transforms/GlobalOpt/metadata.ll
+++ b/llvm/test/Transforms/GlobalOpt/metadata.ll
@@ -17,8 +17,8 @@
 ; for 'MetadataAsValue::get(ValueAsMetadata::get(Value*))'.  When @G drops to
 ; null, the ValueAsMetadata instance gets replaced by metadata !{}, or
 ; MDNode::get({}).
-  call void @llvm.foo(metadata !{i8*** @G}, metadata !{i32 %x})
-; CHECK: call void @llvm.foo(metadata ![[EMPTY:[0-9]+]], metadata !{i32 %x})
+  call void @llvm.foo(metadata i8*** @G, metadata i32 %x)
+; CHECK: call void @llvm.foo(metadata ![[EMPTY:[0-9]+]], metadata i32 %x)
   ret void
 }
 
@@ -27,6 +27,6 @@
 !named = !{!0}
 ; CHECK: !named = !{![[NULL:[0-9]+]]}
 
-!0 = metadata !{i8*** @G}
-; CHECK-DAG: ![[NULL]] = metadata !{null}
-; CHECK-DAG: ![[EMPTY]] = metadata !{}
+!0 = !{i8*** @G}
+; CHECK-DAG: ![[NULL]] = !{null}
+; CHECK-DAG: ![[EMPTY]] = !{}