[PPC64] Support "symbol@high" and "symbol@higha" symbol modifers.

Add support for the "@high" and "@higha" symbol modifiers in powerpc64 assembly.
The modifiers represent accessing the segment consiting of bits 16-31 of a
64-bit address/offset.

Differential Revision: https://reviews.llvm.org/D47729

llvm-svn: 334855
diff --git a/llvm/lib/MC/MCExpr.cpp b/llvm/lib/MC/MCExpr.cpp
index f2d078b..c4e8fb6 100644
--- a/llvm/lib/MC/MCExpr.cpp
+++ b/llvm/lib/MC/MCExpr.cpp
@@ -239,6 +239,8 @@
   case VK_PPC_LO: return "l";
   case VK_PPC_HI: return "h";
   case VK_PPC_HA: return "ha";
+  case VK_PPC_HIGH: return "high";
+  case VK_PPC_HIGHA: return "higha";
   case VK_PPC_HIGHER: return "higher";
   case VK_PPC_HIGHERA: return "highera";
   case VK_PPC_HIGHEST: return "highest";
@@ -343,6 +345,8 @@
     .Case("l", VK_PPC_LO)
     .Case("h", VK_PPC_HI)
     .Case("ha", VK_PPC_HA)
+    .Case("high", VK_PPC_HIGH)
+    .Case("higha", VK_PPC_HIGHA)
     .Case("higher", VK_PPC_HIGHER)
     .Case("highera", VK_PPC_HIGHERA)
     .Case("highest", VK_PPC_HIGHEST)