Enhance boolean simplification to handle 16-/64-bit RDRAND

- RDRAND always clears the destination value when a random value is not
  available (i.e. CF == 0). This value is truncated or zero-extended as
  the false boolean value to be returned. Boolean simplification needs
  to skip this 'zext' or 'trunc' node.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178312 91177308-0d34-0410-b5e6-96231b3b80d8