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/rand-distrib-1.ispc

@@ -11,7 +11,7 @@ export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
     for (uniform int i = 0; i < iters; ++i) {
         unsigned int val = random(&state);
         for (uniform int j = 0; j < 32; ++j) {
-            if (val & (1<<j))
+            if (val & (1ul<<j))
                 ++count[j];
         }
     }