improve column location for binary const exprs
Parser: enforce a newline after each instruction
add error handling to precondition boolean preds
Enforce all arguments of LLVMBoolPred to have the same type
Disallow creation of new identifiers in the precondition
Allow expressions as function/predicate arguments
Add predicate MaskedValueIsZero
Add function trunc
Allow named types for binary operation instructions.

This weakens the type check in BinOp.__init__. This shouldn't create problems, since the front end still requires ensureIntType().