[TableGen] Handle ValueType in CodeGenDAGPatterns GetNumNodeResults

A ValueType in a pattern dag is a type cast, and GetNumNodeResults should
handle it (the type cast has only one result).

This comes up, for example, during the type checking of pattern fragments, for
example, AArch64's Neon_combine_2d fragment is:
  dag Operands = (ops node:$Rm, node:$Rn);
  dag Fragment = (v2f64 (concat_vectors (v1f64 node:$Rm), (v1f64 node:$Rn)));

llvm-svn: 198347