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/soa-3.ispc

@@ -1,4 +1,4 @@
-// syntax error, unexpected '-', expecting int32 constant
+// syntax error, unexpected '-', expecting int
 
 struct F { float a, b, c; };