TooManyLiveRegs check: fix break that was broken by allocator change.

In #196, various `break`s and early returns` out of a main allocator
loop were changed to pass through a point that returned scratch storage
to a context for later reuse.

Unfortunately, this refactor introduced a bug: a `break` became a `break
'outer`, and the latter meant that we were skipping some checks.

Fortunately this only happens in a code-path where the given allocation
problem has too many constraints, and it means that we get a panic
rather than a `TooManyLiveRegs` error cleanly returned, so this is not a
security issue. However, it has me a little paranoid and we should
carefully re-audit #196's changes when we get a chance.

(Discovered while making changes to ABI code that caused too many defs
constrained to regs, resulting in an un-allocatable input; not reachable
from mainline Cranelift.)

Author: cfallin

src/ion/process.rs

@@ -1258,8 +1258,7 @@ impl<'a, F: Function> Env<'a, F> {
                                 continue;
                             }
                             if preg_range.from >= range.to {
-                                // Success, return scratch memory to context and finish
-                                break 'outer;
+                                break;
                             }
                             if lr.is_valid() {
                                 if self.minimal_bundle(self.ranges[*lr].bundle) {