Improvements to constant folding.

We can now do constant folding with all basic datatypes (the previous
implementation handled int32 well, but had limited, if any, coverage
for other datatypes.)

Reduced a bit of repeated code in the constant folding implementation
through template helper functions.

tests_errors/const-too-large.ispc

@@ -0,0 +1,3 @@
+// Binary expression with type "const uniform int8" can't represent value. 
+
+int8 v = (int8)64 * (int8)32;