Modify rules for default variability of pointed-to types.

Now, the pointed-to type is always uniform by default (if an explicit
rate qualifier isn't provided).  This rule is easier to remember and
seems to work well in more cases than the previous rule from 6d7ff7eba2.

stdlib.ispc

@@ -446,8 +446,8 @@ count_trailing_zeros(int64 v) {
 // AOS/SOA conversion
 
 static inline void
-aos_to_soa3(uniform float a[], float * uniform v0, float * uniform v1, 
-            float * uniform v2) {
+aos_to_soa3(uniform float a[], varying float * uniform v0, 
+            varying float * uniform v1, varying float * uniform v2) {
     __aos_to_soa3_float(a, v0, v1, v2);
 }
 
@@ -457,8 +457,9 @@ soa_to_aos3(float v0, float v1, float v2, uniform float a[]) {
 }
 
 static inline void
-aos_to_soa4(uniform float a[], float * uniform v0, float * uniform v1, 
-            float * uniform v2, float * uniform v3) {
+aos_to_soa4(uniform float a[], varying float * uniform v0,
+            varying float * uniform v1, varying float * uniform v2,
+            varying float * uniform v3) {
     __aos_to_soa4_float(a, v0, v1, v2, v3);
 }
 
@@ -468,10 +469,10 @@ soa_to_aos4(float v0, float v1, float v2, float v3, uniform float a[]) {
 }
 
 static inline void
-aos_to_soa3(uniform int32 a[], int32 * uniform v0, int32 * uniform v1, 
-            int32 * uniform v2) {
-    aos_to_soa3((uniform float * uniform)a, (float * uniform)v0, 
-                (float * uniform)v1, (float * uniform)v2);
+aos_to_soa3(uniform int32 a[], varying int32 * uniform v0,
+            varying int32 * uniform v1, varying int32 * uniform v2) {
+    aos_to_soa3((uniform float * uniform)a, (varying float * uniform)v0, 
+                (varying float * uniform)v1, (varying float * uniform)v2);
 }
 
 static inline void
@@ -481,11 +482,12 @@ soa_to_aos3(int32 v0, int32 v1, int32 v2, uniform int32 a[]) {
 }
 
 static inline void
-aos_to_soa4(uniform int32 a[], int32 * uniform v0, int32 * uniform v1, 
-            int32 * uniform v2, int32 * uniform v3) {
-    aos_to_soa4((uniform float * uniform)a, (float * uniform )v0, 
-                (float * uniform)v1, (float * uniform)v2, 
-                (float * uniform)v3);
+aos_to_soa4(uniform int32 a[], varying int32 * uniform v0, 
+            varying int32 * uniform v1, varying int32 * uniform v2, 
+            varying int32 * uniform v3) {
+    aos_to_soa4((uniform float * uniform)a, (varying float * uniform )v0, 
+                (varying float * uniform)v1, (varying float * uniform)v2, 
+                (varying float * uniform)v3);
 }
 
 static inline void
@@ -765,7 +767,7 @@ static unsigned int64 exclusive_scan_or(unsigned int64 v) {
 
 static inline uniform int 
 packed_load_active(uniform unsigned int a[],
-                   unsigned int * uniform vals) {
+                   varying unsigned int * uniform vals) {
     return __packed_load_active(a, vals, (UIntMaskType)__mask);
 }
 
@@ -776,7 +778,7 @@ packed_store_active(uniform unsigned int a[],
 }
 
 static inline uniform int 
-packed_load_active(uniform int a[], int * uniform vals) {
+packed_load_active(uniform int a[], varying int * uniform vals) {
     return __packed_load_active(a, vals, (IntMaskType)__mask);
 }