[turbofan] Fix optimized lowering of Math.imul.

src/compiler/typer.cc

Index: src/compiler/typer.cc
diff --git a/src/compiler/typer.cc b/src/compiler/typer.cc
index 0a96dc743409cadd1e7f80522ee6f7bbed7e1ec9..75628517331e9d6562ae8e4377404b3db88550a7 100644
--- a/src/compiler/typer.cc
+++ b/src/compiler/typer.cc
@@ -1745,6 +1745,8 @@ Type* Typer::Visitor::TypeNumberShiftRightLogical(Node* node) {
   return Type::Unsigned32();
 }
 
+Type* Typer::Visitor::TypeNumberImul(Node* node) { return Type::Signed32(); }
+
 Type* Typer::Visitor::TypeNumberClz32(Node* node) {
   return typer_->cache_.kZeroToThirtyTwo;
 }