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/lib/IR/AsmWriter.cpp b/llvm/lib/IR/AsmWriter.cpp
index 8fe03c0..6153e35 100644
--- a/llvm/lib/IR/AsmWriter.cpp
+++ b/llvm/lib/IR/AsmWriter.cpp
@@ -1264,7 +1264,6 @@
       Out << ' ';
       WriteAsOperandInternal(Out, V, TypePrinter, Machine, Context);
     } else {
-      Out << "metadata ";
       WriteAsOperandInternal(Out, MD, TypePrinter, Machine, Context);
     }
     if (mi + 1 != me)
@@ -1381,13 +1380,9 @@
   assert((FromValue || !isa<LocalAsMetadata>(V)) &&
          "Unexpected function-local metadata outside of value argument");
 
-  if (FromValue)
-    Out << "!{";
   TypePrinter->print(V->getValue()->getType(), Out);
   Out << ' ';
   WriteAsOperandInternal(Out, V->getValue(), TypePrinter, Machine, Context);
-  if (FromValue)
-    Out << "}";
 }
 
 void AssemblyWriter::init() {
@@ -2378,7 +2373,7 @@
 }
 
 void AssemblyWriter::writeMDNode(unsigned Slot, const MDNode *Node) {
-  Out << '!' << Slot << " = metadata ";
+  Out << '!' << Slot << " = ";
   printMDNodeBody(Node);
 }