tree-object-size.cc: Fix assert constant offset in check_for_plus_in_loops [PR122012]

tree-object-size.cc: Fix assert constant offset in check_for_plus_in_loops [PR122012]

After commit 51b85dfeb19652bf3e0aaec08828ba7cee1e641c, when the
pointer offset is a variable in the loop, the object size of the
pointer may also need to be reexamined.
Which make gcc_assert in the check_for_plus_in_loops failed.

gcc/ChangeLog:

	PR tree-optimization/122012
	* tree-object-size.cc (check_for_plus_in_loops): Skip check
	for the variable offset

gcc/testsuite/ChangeLog:

	PR tree-optimization/122012
	* gcc.dg/torture/pr122012.c: New test.

Signed-off-by: Linsen Zhou <i@lin.moe>