Add wi_fold_in_parts.

Add wi_fold_in_parts.

range-ops uses wi_fold to individually fold subranges one at a time and
then combined them.  This patch first calls wi_fold_in_parts which checks if
one of the subranges is small, and if so, further splits that subrange
into constants.

	gcc/
	PR tree-optimization/96542
	* range-op.cc (range_operator::wi_fold_in_parts): New.
	(range_operator::fold_range): Call wi_fold_in_parts.
	(operator_lshift::wi_fold): Fix broken lshift by [0,0].
	* range-op.h (wi_fold_in_parts): Add prototype.

	gcc/testsuite
	* gcc.dg/pr96542.c: New.