6646019: array subscript expressions become top() with -d64
Summary: avoid failures while compiling not-taken paths
Reviewed-by:

Several "Tester" programs fail (in different ways) with -d64 when a
negative array length or negative subscript is used. C2 proves that
the subscript expression does not meet the array bounds and a
ConvI2LNode construction returns top. The top node can cause problems
downstream with a variety of assertion failures or crashes.

The fix in parse2.cpp preemptively handles the cases when the array
bound is negative. The CmpU bounds check trick doesn't work if the
array bound is negative. However, if the parser observes the bound is
negative, then it can proceed as if it is 0, and the exiting mechanism
causes the path to disappear into an uncommon_trap.