Implement ranger folder for __builtin_signbit.

Now that we keep track of the signbit, we can use it to fold __builtin_signbit.

I am assuming I don't have try too hard to get the actual signbit
number and 1 will do.  Especially, since we're inconsistent in trunk whether
we fold the builtin or whether we calculate it at runtime.

abulafia:~$ cat a.c
float nzero = -0.0;

main(){
    printf("0x%x\n", __builtin_signbit(-0.0));
    printf("0x%x\n", __builtin_signbit(nzero));
}
abulafia:~$ gcc a.c -w && ./a.out
0x1
0x80000000

It is amazing that we've been failing to fold something as simple as
this:

	if (x > 5.0)
	  num = __builtin_signbit (x);

It does the right thing now :-P.

gcc/ChangeLog:

	* gimple-range-fold.cc
	(fold_using_range::range_of_builtin_int_call): Add case for
	CFN_BUILT_IN_SIGNBIT.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/vrp-float-signbit-1.c: New test.