Choose type for integer literals to match the target mask size (if possible).

On a target with a 16-bit mask (for example), we would choose the type
of an integer literal "1024" to be an int16.  Previously, we used an int32,
which is a worse fit and leads to less efficient code than an int16
on a 16-bit mask target.  (However, we'd still give an integer literal
1000000 the type int32, even in a 16-bit target.)

Updated the tests to still pass with 8 and 16-bit targets, given this
change.

tests/atomics-4.ispc

@@ -5,10 +5,10 @@ uniform int32 s = 0;
 
 export void f_f(uniform float RET[], uniform float aFOO[]) {
     float a = aFOO[programIndex]; 
-    float b = atomic_or_global(&s, (1<<min(programIndex,30)));
+    float b = atomic_or_global(&s, (1ul<<min(programIndex,30)));
     RET[programIndex] = s;
 }
 
 export void result(uniform float RET[]) {
-    RET[programIndex] = (1<<min(programCount,31))-1;
+    RET[programIndex] = (1ul<<min(programCount,31))-1;
 }