VR-VALUES [PR28794]: optimize compare assignments also

VR-VALUES [PR28794]: optimize compare assignments also

This patch fixes the oldish (2006) bug where VRP was not
optimizing the comparison for assignments while handling
them for GIMPLE_COND only.
It just happens to also solves PR 103281 due to allowing
to optimize `c < 1` to `c == 0` and then we get
`(c == 0) == c` (which was handled by r14-2501-g285c9d04).

OK? Bootstrapped and tested on x86_64-linux-gnu with no
regressions.

	PR tree-optimization/103281
	PR tree-optimization/28794

gcc/ChangeLog:

	* vr-values.cc (simplify_using_ranges::simplify_cond_using_ranges_1): Split out
	majority to ...
	(simplify_using_ranges::simplify_compare_using_ranges_1): Here.
	(simplify_using_ranges::simplify_casted_cond): Rename to ...
	(simplify_using_ranges::simplify_casted_compare): This
	and change arguments to take op0 and op1.
	(simplify_using_ranges::simplify_compare_assign_using_ranges_1): New method.
	(simplify_using_ranges::simplify): For tcc_comparison assignments call
	simplify_compare_assign_using_ranges_1.
	* vr-values.h (simplify_using_ranges): Add
	new methods, simplify_compare_using_ranges_1 and simplify_compare_assign_using_ranges_1.
	Rename simplify_casted_cond and simplify_casted_compare and
	update argument types.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/pr103281-1.c: New test.
	* gcc.dg/tree-ssa/vrp-compare-1.c: New test.