Use native float/half conversion instructions with the AVX2 target.

tests/half-3.ispc

@@ -0,0 +1,21 @@
+
+export uniform int width() { return programCount; }
+
+export void f_v(uniform float RET[]) {
+    int errors = 0;
+
+    foreach (i = 0 ... 65535) {
+        unsigned int16 h = i;
+        float f = half_to_float(i);
+        h = float_to_half(f);
+
+        int mismatches = (f == f && i != h);
+        errors += reduce_add(mismatches);
+    }
+
+    RET[programIndex] = errors;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 0;
+}