Add support for int8/int16 types. Addresses issues #9 and #42.

tests/op-plus-equals-ensure-one-lhs-eval.ispc

@@ -4,7 +4,7 @@ export uniform int width() { return programCount; }
 export void f_fu(uniform float ret[], uniform float aa[], uniform float b) {
     uniform float foo[16];
     for (uniform int i = 0; i < 16; ++i)
-        foo[i] = 1;
+        foo[i] = i;
 
     uniform int i = 0;
     foo[i++] += 1;