[Sema,CodeGen] Remove comment from SemaChecking about a builtin_shufflevector...

[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