re PR tree-optimization/38789 (__builtin_constant_p appears to evaluate to true for non-constant arguments.)

	PR tree-optimization/38789
	* tree-ssa-threadedge.c
	(record_temporary_equivalences_from_stmts_at_dest): Ignore calls to
	__builtin_constant_p.

	* gcc.c-torture/compile/pr38789.c: New test.

From-SVN: r143435