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/initexpr-2.ispc

@@ -1,3 +1,3 @@
-// Initializer list for array "varying int32[2][4]" must have no more than 2 elements (has 3)
+// Initializer list for array "varying int32\[2\]\[4\]" must have no more than 2 elements \(has 3\)
 
 int a[2][4] = { { 1, 2, 3 }, { 1, 2, 3, 4 }, 1 };