Add more tests of basic gather functionality.

tests/gather-float-5.ispc

@@ -0,0 +1,19 @@
+
+export uniform int width() { return programCount; }
+
+int zero = 0;
+void *gptr;
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    uniform float a[programCount];
+    for (uniform int i = 0; i < programCount; ++i)
+        a[i] = aFOO[i];
+
+    float *ptr = (aFOO[0] == 1234) ? (float * varying)gptr : (a + programIndex);
+    int g = *ptr;
+    RET[programIndex] = g; 
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1 + programIndex;
+}