Revert recent changes to value_bits_valid.

  http://sourceware.org/ml/gdb-patches/2013-07/msg00243.html

  * value.c (value_bits_valid): Revert previous change, and change
  by Pedro on 2013-07-04, due to regressions in
  gdb.dwarf2/implptr.exp and gdb.dwarf2/pieces.exp.