[turbofan]: Fix bug in register hinting

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 4a222a43bef66c2c22008e2a5ddbb8d2113f6a11..28c99b95544ba4e7c3839730f5e4ab117f0e3087 100644
--- a/src/compiler/register-allocator.cc
+++ b/src/compiler/register-allocator.cc
@@ -196,8 +196,11 @@ void UsePosition::ResolveHint(UsePosition* use_pos) {
 
 void UsePosition::set_type(UsePositionType type, bool register_beneficial) {
   DCHECK_IMPLIES(type == UsePositionType::kRequiresSlot, !register_beneficial);
+  DCHECK_EQ(kUnassignedRegister, AssignedRegisterField::decode(flags_));
   flags_ = TypeField::encode(type) |
-           RegisterBeneficialField::encode(register_beneficial);
+           RegisterBeneficialField::encode(register_beneficial) |
+           HintTypeField::encode(HintTypeField::decode(flags_)) |
+           AssignedRegisterField::encode(kUnassignedRegister);
 }