Fold comparisons between sqrt and zero

Fold comparisons between sqrt and zero

The expression:

      signbit(sqrt(x))

is always 0 for -ffast-math.  The signbit fold first converts it to:

      sqrt(x) < 0

and whether we realise that this is false depends on a race between two
folders: the sqrt comparison folder, which wants to convert it to

      x < 0*0

and the generic tree_expr_nonnegative_p rule for ... < 0, which would
give the hoped-for 0.

The sqrt code already handles comparisons with negative values specially,
so this patch simply extends that idea to comparisons with zero.

Tested on x86_64-linux-gnu, aarch64-linux-gnu and arm-linux-gnueabi.

gcc/
	* match.pd: Handle sqrt(x) cmp 0 specially.

gcc/testsuite/
	* gcc.dg/torture/builtin-sqrt-cmp-1.c: New test.

From-SVN: r229422