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/machinst/vcode.rs

@@ -20,6 +20,7 @@
 use crate::ir::{self, types, SourceLoc};
 use crate::machinst::*;
 use crate::settings;
+use crate::timing;
 
 use regalloc::Function as RegallocFunction;
 use regalloc::Set as RegallocSet;
@@ -424,6 +425,7 @@ impl<I: VCodeInst> VCode<I> {
     where
         I: MachInstEmit,
     {
+        let _tt = timing::vcode_emit();
         let mut buffer = MachBuffer::new();
         let mut state = I::State::new(&*self.abi);