Return ExprError if both arguments to the mask form of __builtin_shufflvector...

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.

llvm-svn: 187344