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/local-atomics-13.ispc

@@ -7,7 +7,7 @@ export void f_f(uniform float RET[], uniform float aFOO[]) {
     float a = aFOO[programIndex]; 
     int32 b = 0;
     if (programIndex < 28 && (programIndex & 1))
-        b = atomic_or_local(&s, (1 << programIndex));
+        b = atomic_or_local(&s, (1ul << programIndex));
     RET[programIndex] = popcnt(reduce_max(b));
 }