[csmith-project/csmith] a39f3c: fix a bug analyzing union field reads/writes

[Xuejun Yang <jxyang@cs.utah.edu>]

  Branch: refs/heads/master
  Home:   https://github.com/csmith-project/csmith

  Commit: a39f3c2a62c59d11aecd914ad176b6e5b1ab412a
      https://github.com/csmith-project/csmith/commit/a39f3c2a62c59d11aecd914ad176b6e5b1ab412a
  Author: Xuejun Yang <jxyang@cs.utah.edu>
  Date:   2011-08-05 (Fri, 05 Aug 2011)

  Changed paths:
    M src/Effect.cpp
  M src/FactUnion.cpp
  M src/Type.cpp
  M src/Type.h
  M src/Variable.cpp
  M src/Variable.h

  Log Message:
  -----------
  fix a bug analyzing union field reads/writes

correct a wrong assumption: writing to a field in a struct that is in an union does not change the last written field property. It turns out if the field itself is a struct with padding bytes, all of the union field become non-readable!