Hide IVOPTs strip_offset

Hide IVOPTs strip_offset

PR110243 shows strip_offset has some correctness issues, the following
avoids using it from loop distribution which can use the more correct
split_constant_offset from data-ref analysis instead.  The patch then
un-exports the function from IVOPTs.

	* tree-loop-distribution.cc (classify_builtin_st): Use
	split_constant_offset.
	* tree-ssa-loop-ivopts.h (strip_offset): Remove.
	* tree-ssa-loop-ivopts.cc (strip_offset): Make static.