[turbofan] Introduces another DCHECK in the register allocator.

src/compiler/register-allocator.cc

Index: src/compiler/register-allocator.cc
diff --git a/src/compiler/register-allocator.cc b/src/compiler/register-allocator.cc
index 9b01c1515f24cdbcde0e3e4be560b435a69ee56d..6e62b3ad96853f7b64ff821c2c9e9126ff3bac98 100644
--- a/src/compiler/register-allocator.cc
+++ b/src/compiler/register-allocator.cc
@@ -1114,6 +1114,10 @@ void TopLevelLiveRange::AddUseInterval(LifetimePosition start,
       interval->set_next(first_interval_);
       first_interval_ = interval;
     } else {
+      // Order of instruction's processing (see ProcessInstructions) guarantees
+      // that each new use interval either precedes, intersects with or touches
+      // the last added interval.
+      DCHECK(start <= first_interval_->end());
       first_interval_->set_start(Min(start, first_interval_->start()));
       first_interval_->set_end(Max(end, first_interval_->end()));
     }