middle-end/110495 - avoid associating constants with (VL) vectors

middle-end/110495 - avoid associating constants with (VL) vectors

When trying to associate (v + INT_MAX) + INT_MAX we are using
the TREE_OVERFLOW bit to check for correctness.  That isn't
working for VECTOR_CSTs and it can't in general when one considers
VL vectors.  It looks like it should work for COMPLEX_CSTs but
I didn't try to single out _Complex int in this change.

The following makes sure that for vectors we use the fallback of
using unsigned arithmetic when associating the above to
v + (INT_MAX + INT_MAX).

	PR middle-end/110495
	* tree.h (TREE_OVERFLOW): Do not mention VECTOR_CSTs
	since we do not set TREE_OVERFLOW on those since the
	introduction of VL vectors.
	* match.pd (x +- CST +- CST): For VECTOR_CST do not look
	at TREE_OVERFLOW to determine validity of association.

	* gcc.dg/tree-ssa/addadd-2.c: Amend.
	* gcc.dg/tree-ssa/forwprop-27.c: Adjust.