[turbofan] Improve typing for string::length property.

src/compiler/access-builder.cc

Index: src/compiler/access-builder.cc
diff --git a/src/compiler/access-builder.cc b/src/compiler/access-builder.cc
index 8c8e53092b81f07dbd442b1c286a837298b1d679..589ae65366579070406cb083feefb773cdce9891 100644
--- a/src/compiler/access-builder.cc
+++ b/src/compiler/access-builder.cc
@@ -61,7 +61,8 @@ FieldAccess AccessBuilder::ForMapInstanceType() {
 // static
 FieldAccess AccessBuilder::ForStringLength() {
   return {kTaggedBase, String::kLengthOffset, Handle<Name>(),
-          Type::SignedSmall(), kMachAnyTagged};
+          Type::Intersect(Type::UnsignedSmall(), Type::TaggedSigned()),
+          kMachAnyTagged};
 }