Add support for running the regalloc2 checker. (#4043)

With these fixes, all this PR has to do is instantiate and run the
checker on the `regalloc2::Output`. This is off by default, and is
enabled by setting the `regalloc_checker` Cranelift option.

This restores the old functionality provided by e.g. the
`backtracking_checked` regalloc algorithm setting rather than
`backtracking` when we were still on regalloc.rs.

cranelift/codegen/src/timing.rs

@@ -65,6 +65,7 @@ define_passes! {
     vcode_emit_finish: "VCode emission finalization",
 
     regalloc: "Register allocation",
+    regalloc_checker: "Register allocation symbolic verification",
     binemit: "Binary machine code emission",
     layout_renumber: "Layout full renumbering",