Switch to unordered floating point compares.

In particular, this gives us desired behavior for NaNs (all compares
involving a NaN evaluate to true).  This in turn allows writing the
canonical isnan() function as "v != v".

Added isnan() to the standard library as well.

tests/half-3.ispc

@@ -9,7 +9,7 @@ export void f_v(uniform float RET[]) {
         float f = half_to_float(i);
         h = float_to_half(f);
 
-        int mismatches = (f == f && i != h);
+        int mismatches = (!isnan(f) && i != h);
         if (any(mismatches != 0))
             print("mismatch: orig int16 % -> float % -> half %\n", i, f, h);
         errors += reduce_add(mismatches);