Dear all,

poor error recovery while trying to simplify intrinsics with given
invalid arguments seems to be a recurrent theme in testcases submitted
by Gerhard.  In the present case, simplification of PACK() chokes on
the array argument being a bad decl.

The most general approach that came to my mind is to modify function
is_constant_array_expr: when the declared shape of the array indicates
a size greater than zero, but the constructor is missing or empty,
then something bad may have happened, and the array cannot be
considered constant.  We thus punt on simplification of something
that cannot be simplified.  With some luck, this might prevent issues
in similar cases elsewhere...

Regtested on x86_64-pc-linux-gnu.  OK for mainline?

Thanks,
Harald