fold-const (fold_binary): Also optimize a/cbrt(b/c) into a*cbrt(c/b) if flag_unsafe_math_optimizations is set.

	* fold-const (fold_binary) [RDIV_EXPR]: Also optimize a/cbrt(b/c)
	into a*cbrt(c/b) if flag_unsafe_math_optimizations is set.

testuite/ChangeLog:

	* gcc.dg/builtins-11.c: Also check folding of a/cbrt(b/c).

From-SVN: r125641