tree-optimization/107618 - enhance copy propagation of constants

tree-optimization/107618 - enhance copy propagation of constants

The following enhances copy propagation of constants to also see
through simple operations like conversions but also operations with
otherwise constant operands.  That's required to fulfill the promise

      /* Copy propagation also copy-propagates constants, this is necessary
         to forward object-size and builtin folding results properly.  */
      NEXT_PASS (pass_copy_prop);

and avoid false diagnostics as shown in the testcase.  We're
using gimple_fold_stmt_to_constant_1 with not following SSA edges
and accordingly adjust what stmts we simulate during SSA propagation.

	PR tree-optimization/107618
	* tree-ssa-copy.cc (stmt_may_generate_copy): Simulate all
	assignments with a single SSA use.
	(copy_prop_visit_assignment): Use gimple_fold_stmt_to_constant_1
	to perform simple constant folding.
	(copy_prop::visit_stmt): Visit all assignments.

	* gcc.dg/pr107618.c: New testcase.