added tests for rcp/rsqrt double

tests/test-148.ispc

@@ -0,0 +1,16 @@
+
+export uniform int width() { return programCount; }
+
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    double x = aFOO[programIndex]; 
+    double d, invsqrt = rsqrt(x);
+    d = (x * (invsqrt * invsqrt)) - 1.0d0;
+    if (d < 0.0d0) d = -d;
+    RET[programIndex] = d > 1d-5;
+}
+
+
+export void result(uniform float RET[]) {
+       RET[programIndex] = 0;
+}