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_errors/int-ptr-fail.ispc

@@ -1,5 +1,5 @@
-// Type conversion from "const uniform int32" to "uniform int32 * varying" for initializer is not possible
+// Type conversion from "const uniform int32" to "uniform int32 \* varying" for initializer is not possible
 
 int voo() {
-    int * varying foo = 1;
+    int * varying foo = 1l;
 }