tree-optimization/126171 - SCEV and minus of INT_MIN

The following fixes the existing mitigation against UB for handling
of subtraction of the most negative value in a type.  We've only
guarded the negation itself which is effective for avoiding UB in
that but it does not help in fixing a wrong scev_direction that
results from flipping the operation from minus to plus.  So the
patch, instead of only promoting the negation to unsigned, promotes
the whole operation.

	PR tree-optimization/126171
	* tree-scalar-evolution.cc (scev_dfs::add_to_evolution):
	Remove code argument.  Remove MINUS_EXPR handling here.
	(scev_dfs::follow_ssa_edge_binary): Likewise.
	(scev_dfs::follow_ssa_edge_expr): Handle MINUS_EXPR
	separately here, promoting the whole operation to unsigned
	instead of just the addend.

	* gcc.dg/pr126171-1.c: New testcase.
	* gcc.dg/torture/pr126171-2.c: Likewise.