Type checking and Dominator Tree integrity checks in Verifier (#66)

* Verify that a recomputed dominator tree is identical to the existing one.
* The verifier now typechecks instruction results and arguments.
* The verifier now typechecks instruction results and arguments.
* The verifier now typechecks instruction results and arguments.
* Added `inst_{fixed,variable}_args` accessor functions.
* Improved error messages in verifier.
* Type check return statements against the function signature.

lib/cretonne/src/ir/instructions.rs

@@ -398,7 +398,6 @@ pub struct OpcodeConstraints {
     /// Offset into `OPERAND_CONSTRAINT` table of the descriptors for this opcode. The first
     /// `fixed_results()` entries describe the result constraints, then follows constraints for the
     /// fixed `Value` input operands. (`fixed_value_arguments()` of them).
-    /// format.
     constraint_offset: u16,
 }