cmd/compile: do regalloc check only when checkEnabled

No point doing this check all the time.

Fixes #15621

Change-Id: I1966c061986fe98fe9ebe146d6b9738c13cef724
Reviewed-on: https://go-review.googlesource.com/30670Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/ssa/regalloc.go

@@ -1436,8 +1436,7 @@ func (s *regAllocState) regalloc(f *Func) {
 		}
 		s.endRegs[b.ID] = regList
 
-		// Check. TODO: remove
-		{
+		if checkEnabled {
 			liveSet.clear()
 			for _, x := range s.live[b.ID] {
 				liveSet.add(x.ID)