Add count_{leading,trailing}_zeros() functions to stdlib.

(Documentation is still yet to be written.)

tests/count-leading-trailing-zeros-1.ispc

@@ -0,0 +1,11 @@
+
+export uniform int width() { return programCount; }
+
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    RET[programIndex] = count_trailing_zeros(0xf0);
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 4;
+}