tree-optimization/118954 - avoid UB on ref created by predcom

tree-optimization/118954 - avoid UB on ref created by predcom

When predicitive commoning moves an invariant ref it makes sure to
not build a MEM_REF with a base that is negatively offsetted from
an object.  But in trying to preserve some transforms it does not
consider association of a constant offset with the address computation
in DR_BASE_ADDRESS leading to exactly this problem again.  This is
arguably a problem in data-ref analysis producing such an out-of-bound
DR_BASE_ADDRESS, but this looks quite involved to fix, so the
following avoids the association in one more case.  This fixes the
testcase while preserving the desired transform in
gcc.dg/tree-ssa/predcom-1.c.

	PR tree-optimization/118954
	* tree-predcom.cc (ref_at_iteration): Make sure to not
	associate the constant offset with DR_BASE_ADDRESS when
	that is an offsetted pointer.

	* gcc.dg/torture/pr118954.c: New testcase.