AMDGPU: Add 64-bit relative variant kind

Differential Revision: https://reviews.llvm.org/D47601

llvm-svn: 334443
diff --git a/llvm/lib/MC/MCExpr.cpp b/llvm/lib/MC/MCExpr.cpp
index d38127f..f2d078b 100644
--- a/llvm/lib/MC/MCExpr.cpp
+++ b/llvm/lib/MC/MCExpr.cpp
@@ -303,6 +303,7 @@
   case VK_AMDGPU_GOTPCREL32_HI: return "gotpcrel32@hi";
   case VK_AMDGPU_REL32_LO: return "rel32@lo";
   case VK_AMDGPU_REL32_HI: return "rel32@hi";
+  case VK_AMDGPU_REL64: return "rel64";
   }
   llvm_unreachable("Invalid variant kind");
 }
@@ -410,6 +411,7 @@
     .Case("gotpcrel32@hi", VK_AMDGPU_GOTPCREL32_HI)
     .Case("rel32@lo", VK_AMDGPU_REL32_LO)
     .Case("rel32@hi", VK_AMDGPU_REL32_HI)
+    .Case("rel64", VK_AMDGPU_REL64)
     .Default(VK_Invalid);
 }