[turbofan] Better narrow the derived type for the right shift operation.

src/compiler/typer.cc

Index: src/compiler/typer.cc
diff --git a/src/compiler/typer.cc b/src/compiler/typer.cc
index 4b3a8830a0fff9cbed6694b9588742225d262814..b63452f6cb31e004da08a2d5e3f2ae80d5e0121b 100644
--- a/src/compiler/typer.cc
+++ b/src/compiler/typer.cc
@@ -916,11 +916,15 @@ Type* Typer::Visitor::JSShiftRightTyper(Type* lhs, Type* rhs, Typer* t) {
     // Right-shifting a non-negative value cannot make it negative, nor larger.
     min = std::max(min, 0.0);
     max = std::min(max, lhs->Max());
+    if (rhs->Min() > 0 && rhs->Max() <= 31)

[Benedikt Meurer, 2015/01/26 07:23:47]

Nit: please add { and }.

+      max = static_cast<int>(max) >> static_cast<int>(rhs->Min());
   }
   if (lhs->Max() < 0) {
     // Right-shifting a negative value cannot make it non-negative, nor smaller.
     min = std::max(min, lhs->Min());
     max = std::min(max, -1.0);
+    if (rhs->Min() > 0 && rhs->Max() <= 31)

[Benedikt Meurer, 2015/01/26 07:23:47]

Nit: please add { and }.

+      min = static_cast<int>(min) >> static_cast<int>(rhs->Min());
   }
   if (rhs->Min() > 0 && rhs->Max() <= 31) {
     // Right-shifting by a positive value yields a small integer value.