Return ExprError if both arguments to the mask form of __builtin_shufflvector don't have the same number of elements or the mask isn't an integer vector.

Previously a diagnostic was issued, but the code went ahead and built the ShuffleVectorExpr. While I'm here also simplify a couple lines by wrapping the return ExprError around the Diag calls.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187344 91177308-0d34-0410-b5e6-96231b3b80d8