Fix fuzzbug related to bundle priority ordering.

Changes in computation of bundle priorities during review of the initial
PR introduced a possible mis-ordering of priorities: inner-loop bundle
use weights could exceed the weights of 1_000_000 and 2_000_000 used for
minimal bundles without and with fixed uses (respectively). These two
kinds of minimal bundle are meant to be the highest-priority bundles,
evicting any other bundle they need to, because they can't be split
further. This PR introduces two special bundle weights for these two
kinds of bundles, and clamps all other bundle weights to just below
them.

Thanks to @Amanieu for reporting the issue! Fixes #19.

src/checker.rs

@@ -78,7 +78,7 @@
 
 use crate::{
     Allocation, AllocationKind, Block, Edit, Function, Inst, InstPosition, Operand,
-    OperandConstraint, OperandKind, OperandPos, Output, PReg, ProgPoint, RegClass, VReg,
+    OperandConstraint, OperandKind, OperandPos, Output, PReg, ProgPoint, VReg,
 };
 
 use std::collections::{HashMap, HashSet, VecDeque};