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/ArgumentPromotion/dbg.ll b/llvm/test/Transforms/ArgumentPromotion/dbg.ll
index d155750..dfc67d6 100644
--- a/llvm/test/Transforms/ArgumentPromotion/dbg.ll
+++ b/llvm/test/Transforms/ArgumentPromotion/dbg.ll
@@ -19,8 +19,8 @@
 !llvm.module.flags = !{!0}
 !llvm.dbg.cu = !{!3}
 
-!0 = metadata !{i32 2, metadata !"Debug Info Version", i32 2}
-!1 = metadata !{i32 8, i32 0, metadata !2, null}
-!2 = metadata !{metadata !"0x2e\00test\00test\00\003\001\001\000\006\00256\000\003", null, null, null, null, void (i32**)* @test, null, null, null} ; [ DW_TAG_subprogram ]
-!3 = metadata !{metadata !"0x11\004\00clang version 3.5.0 \000\00\000\00\002", null, null, null, metadata !4, null, null} ; [ DW_TAG_compile_unit ] [/usr/local/google/home/blaikie/dev/scratch/pr20038/reduce/<stdin>] [DW_LANG_C_plus_plus]
-!4 = metadata !{metadata !2}
+!0 = !{i32 2, !"Debug Info Version", i32 2}
+!1 = !{i32 8, i32 0, !2, null}
+!2 = !{!"0x2e\00test\00test\00\003\001\001\000\006\00256\000\003", null, null, null, null, void (i32**)* @test, null, null, null} ; [ DW_TAG_subprogram ]
+!3 = !{!"0x11\004\00clang version 3.5.0 \000\00\000\00\002", null, null, null, !4, null, null} ; [ DW_TAG_compile_unit ] [/usr/local/google/home/blaikie/dev/scratch/pr20038/reduce/<stdin>] [DW_LANG_C_plus_plus]
+!4 = !{!2}