value_bits_valid: Fix latent bug.

Doing something else, I factored out the bits of the value_bits_valid
function that actually handle the check_validity hook, and
surprisingly found out that the result was misbehaving.  Turns out
value_bits_valid has a latent bug.  If the value is not lval_computed,
or doesn't have a check_validity hook, then we should assume the value
is entirely valid, not invalid.  This is currently masked by the
value->optimized_out check -- I ran the testsuite with a gdb_assert(0)
inserted in place of that return being touched by the patch, and it
never triggers.

Tested on x86_64 Fedora 17.

gdb/
2013-07-04  Pedro Alves  <palves@redhat.com>

	* value.c (value_bits_valid): If the value is not lval_computed,
	or doesn't have a check_validity hook, assume the value is entirely
	valid.