tree-optimization/104214 - amend PR100740 fix for pointer compares

tree-optimization/104214 - amend PR100740 fix for pointer compares

When we have a pointer relational compare we have stronger guarantees
about overflow, in particular rewriting BASE0 + STEP0 cmp BASE1 + STEP1
as BASE0 + STEP0 - STEP1 cmp BASE1 is always valid and the new IV0
does not overflow.  The patch basically reverts the previous change
when pointers are involved, keeping only the more conservative handling
for equality compares which can involve comparing different object
addresses.

2022-01-25  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/104214
	* tree-ssa-loop-niter.cc (number_of_iterations_cond): Use
	stronger guarantees for relational pointer compares when
	rewriting BASE0 + STEP0 cmp BASE1 + STEP1 as
	BASE0 + STEP0 - STEP1 cmp BASE1.

	* gcc.dg/vect/pr81196-2.c: New variant testcase only
	requiring vect_int.