[turbofan] Remove overflow check on integer additive op if truncated.

This removes overflow check in addition when we have Smi or
Int32 feedback for the addition, and the result is truncated
to word32. Here is an example, assuming that we passed only
integers to this function so far.

function f(a) { return a + 1 | 0; }

If the result of the addition is word32-truncated, there is no need
for an overflow check.

Until now, we would only eliminate the overflow check
if we knew that the static types of the inputs guaranteed that
the result is in safe integer range. With this CL, we use the
checked type from the feedback, too (but we do not propagate the
truncation!).

Bug: v8:5267,v8:6764
Change-Id: Ia2f929600758f58e6e7db52fef638a0e56c936a9
Reviewed-on: https://chromium-review.googlesource.com/635083
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47668}

src/compiler/simplified-lowering.cc

@@ -1312,11 +1312,13 @@ class RepresentationSelector {
                    Type::Signed32());
       }
       if (lower()) {
-        if (CanOverflowSigned32(node->op(), left_feedback_type,
-                                right_feedback_type, graph_zone())) {
-          ChangeToInt32OverflowOp(node);
-        } else {
+        if (truncation.IsUsedAsWord32() ||
+            !CanOverflowSigned32(node->op(), left_feedback_type,
+                                 right_feedback_type, graph_zone())) {
           ChangeToPureOp(node, Int32Op(node));
+
+        } else {
+          ChangeToInt32OverflowOp(node);
         }
       }
       return;