More AVX fixes

Fix RNG state initialization for 16-wide targets
Fix a number of bugs in reduce_add builtin implementations for AVX.
Fix some tests that had incorrect expected results for the 16-wide
  case.

tests/cwhile-test-60.ispc

@@ -11,5 +11,5 @@ export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
 
 
 export void result(uniform float RET[]) {
-    RET[programIndex] = 10;
+    RET[programIndex] = max(10, 1 + programIndex);
 }