soft-fp64/fsat: Micro-optimize x >= 1 test

Results on the 308 shaders extracted from the fp64 portion of the OpenGL
CTS:

Tiger Lake and Ice Lake had similar results. (Tiger Lake shown)
total instructions in shared programs: 841590 -> 841332 (-0.03%)
instructions in affected programs: 121957 -> 121699 (-0.21%)
helped: 7
HURT: 0
helped stats (abs) min: 15 max: 54 x̄: 36.86 x̃: 41
helped stats (rel) min: 0.16% max: 0.33% x̄: 0.23% x̃: 0.18%
95% mean confidence interval for instructions value: -49.73 -23.98
95% mean confidence interval for instructions %-change: -0.29% -0.16%
Instructions are helped.

total cycles in shared programs: 6926828 -> 6923967 (-0.04%)
cycles in affected programs: 1038569 -> 1035708 (-0.28%)
helped: 7
HURT: 0
helped stats (abs) min: 128 max: 616 x̄: 408.71 x̃: 446
helped stats (rel) min: 0.18% max: 0.44% x̄: 0.29% x̃: 0.22%
95% mean confidence interval for cycles value: -571.72 -245.70
95% mean confidence interval for cycles %-change: -0.38% -0.19%
Cycles are helped.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4142>

src/compiler/glsl/float64.glsl

@@ -264,7 +264,25 @@ __fsat64(uint64_t __a)
    if (__is_nan(__a) || int(a.y) < 0)
       return 0ul;
 
-   if (!__flt64_nonnan(__a, 0x3FF0000000000000ul /* 1.0 */))
+   /* IEEE 754 floating point numbers are specifically designed so that, with
+    * two exceptions, values can be compared by bit-casting to signed integers
+    * with the same number of bits.
+    *
+    * From https://en.wikipedia.org/wiki/IEEE_754-1985#Comparing_floating-point_numbers:
+    *
+    *    When comparing as 2's-complement integers: If the sign bits differ,
+    *    the negative number precedes the positive number, so 2's complement
+    *    gives the correct result (except that negative zero and positive zero
+    *    should be considered equal). If both values are positive, the 2's
+    *    complement comparison again gives the correct result. Otherwise (two
+    *    negative numbers), the correct FP ordering is the opposite of the 2's
+    *    complement ordering.
+    *
+    * We know that both values are not negative, and we know that at least one
+    * value is not zero.  Therefore, we can just use the 2's complement
+    * comparison ordering.
+    */
+   if (ilt64(0x3FF00000, 0x00000000, a.y, a.x))
       return 0x3FF0000000000000ul;
 
    return __a;