Add timing for several new-backend stages.

This PR adds a bit more granularity to the output of e.g. `clif-util
compile -T`, indicating how much time is spent in VCode lowering and
various other new-backend-specific tasks.

cranelift/codegen/src/timing.rs

@@ -64,6 +64,11 @@ define_passes! {
     unreachable_code: "Remove unreachable blocks",
     remove_constant_phis: "Remove constant phi-nodes",
 
+    vcode_lower: "VCode lowering",
+    vcode_post_ra: "VCode post-register allocation finalization",
+    vcode_emit: "VCode emission",
+    vcode_emit_finish: "VCode emission finalization",
+
     regalloc: "Register allocation",
     ra_liveness: "RA liveness analysis",
     ra_cssa: "RA coalescing CSSA",