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/count-leading-trailing-zeros-1.ispc

@@ -3,7 +3,7 @@ export uniform int width() { return programCount; }
 
 
 export void f_f(uniform float RET[], uniform float aFOO[]) {
-    RET[programIndex] = count_trailing_zeros(0xf0);
+    RET[programIndex] = count_trailing_zeros(0xf0ul);
 }
 
 export void result(uniform float RET[]) {