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/settings.rs

@@ -515,6 +515,7 @@ tls_model = "none"
 libcall_call_conv = "isa_default"
 baldrdash_prologue_words = 0
 probestack_size_log2 = 12
+regalloc_checker = false
 enable_verifier = true
 is_pic = false
 use_colocated_libcalls = false