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/Analysis/BlockFrequencyInfo/basic.ll b/llvm/test/Analysis/BlockFrequencyInfo/basic.ll
index 006e6ab..1c24176 100644
--- a/llvm/test/Analysis/BlockFrequencyInfo/basic.ll
+++ b/llvm/test/Analysis/BlockFrequencyInfo/basic.ll
@@ -47,7 +47,7 @@
   ret i32 %result
 }
 
-!0 = metadata !{metadata !"branch_weights", i32 64, i32 4}
+!0 = !{!"branch_weights", i32 64, i32 4}
 
 define i32 @test3(i32 %i, i32 %a, i32 %b, i32 %c, i32 %d, i32 %e) {
 ; CHECK-LABEL: Printing analysis {{.*}} for function 'test3':
@@ -90,7 +90,7 @@
   ret i32 %result
 }
 
-!1 = metadata !{metadata !"branch_weights", i32 4, i32 4, i32 64, i32 4, i32 4}
+!1 = !{!"branch_weights", i32 4, i32 4, i32 64, i32 4, i32 4}
 
 define void @nested_loops(i32 %a) {
 ; CHECK-LABEL: Printing analysis {{.*}} for function 'nested_loops':
@@ -138,4 +138,4 @@
 
 declare void @g(i32)
 
-!2 = metadata !{metadata !"branch_weights", i32 1, i32 4000}
+!2 = !{!"branch_weights", i32 1, i32 4000}