[PowerPC] Implement vec_xxsldwi builtin.

The vec_xxsldwi builtin is missing from altivec.h. This has been requested by
developers working on libvpx for VP9 support for Google.

The patch fixes PR: https://bugs.llvm.org/show_bug.cgi?id=32653
Differential Revision: https://reviews.llvm.org/D33236

llvm-svn: 303766
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index 5973a5f..b794628 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -1697,6 +1697,7 @@
     return SemaBuiltinConstantArgRange(TheCall, 0, 0, 31) ||
            SemaBuiltinConstantArgRange(TheCall, 2, 0, 31);
   case PPC::BI__builtin_vsx_xxpermdi:
+  case PPC::BI__builtin_vsx_xxsldwi:
     return SemaBuiltinVSX(TheCall);
   }
   return SemaBuiltinConstantArgRange(TheCall, i, l, u);