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/coalesce-1.ispc

@@ -2,8 +2,8 @@
 export uniform int width() { return programCount; }
 
 export void f_f(uniform float RET[], uniform float aFOO[]) {
-    uniform float * uniform buf = uniform new uniform float[32*32];
-    for (uniform int i = 0; i < 32*32; ++i)
+    uniform float * uniform buf = uniform new uniform float[32l*32l];
+    for (uniform int i = 0; i < 32l*32l; ++i)
         buf[i] = i;
     
     assert(programIndex <= 64);