add missing bfloat16 repr and improve assertions (#403)

- `BF16TyID` was missing a repr implementation.
- Throw a better exception on impossible casts.
- Add a few assertions. Tested with a debug build.
- Add `pointer_dtype.__str__` to aid kernel debugging.

lib/ir/dispatch.cc

@@ -55,7 +55,7 @@ ir::type *computation_type(ir::type* a_ty, ir::type* b_ty, DivOrMod div_or_mod)
     }
   }
   if (!a_ty->is_integer_ty() || !b_ty->is_integer_ty())
-    throw_unreachable("augment_types");
+    throw_unreachable("computation_type");
   // 4 ) both operands are integer and undergo
   //    integer promotion
   return integer_promote(a_ty, b_ty);
@@ -493,7 +493,7 @@ ir::value *dispatch::cast(ir::value *input, ir::type *dst_ty, ir::builder *build
       other = builder->create_splat(other, src_ty->get_block_shapes());
     return builder->create_icmpNE(input, other);
   }
-  return throw_unreachable("cast");
+  return throw_unreachable("cast from " + src_sca_ty->repr() + " to " + dst_sca_ty->repr());
 }
 
 //===----------------------------------------------------------------------===//

lib/ir/instructions.cc

@@ -232,6 +232,7 @@ icmp_inst::icmp_inst(type *ty, cmp_pred_t pred,
 
 icmp_inst* icmp_inst::create(cmp_pred_t pred, value *lhs, value *rhs, const std::string &name, instruction *next){
   assert(is_int_predicate(pred));
+  assert(lhs->get_type() == rhs->get_type());
   type *res_ty = make_cmp_result_type(lhs->get_type());
   return new icmp_inst(res_ty, pred, lhs, rhs, name, next);
 }
@@ -920,7 +921,5 @@ const constant_int* make_range::get_last() const {
   return last_;
 }
 
-
-
 }
 }