[Sema,CodeGen] Remove comment from SemaChecking about a builtin_shufflevector form that it doesn't support. Remove CodeGen support for the same form since it could never have been used due to the missing support in Sema.

I couldn't find any documentation that this form existed either. Nor is there documentation for one of the remaining two forms, but there is a testcase that uses it.

llvm-svn: 269879
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index 31a20cb..dc91b93 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -3136,8 +3136,7 @@
 
   // Determine which of the following types of shufflevector we're checking:
   // 1) unary, vector mask: (lhs, mask)
-  // 2) binary, vector mask: (lhs, rhs, mask)
-  // 3) binary, scalar mask: (lhs, rhs, index, ..., index)
+  // 2) binary, scalar mask: (lhs, rhs, index, ..., index)
   QualType resType = TheCall->getArg(0)->getType();
   unsigned numElements = 0;