[csa] Re-introduce automatic constant folding for IntPtrAdd and IntPtrSub

src/builtins/builtins-string.cc

Index: src/builtins/builtins-string.cc
diff --git a/src/builtins/builtins-string.cc b/src/builtins/builtins-string.cc
index 2e0fd29cf625cc1cf2760eee303939ea4b216289..9b50d3e40e27ab1251b99476bde2e33614a9a41a 100644
--- a/src/builtins/builtins-string.cc
+++ b/src/builtins/builtins-string.cc
@@ -37,9 +37,9 @@ class StringBuiltinsAssembler : public CodeStubAssembler {
     const int header = SeqOneByteString::kHeaderSize - kHeapObjectTag;
     Node* offset = index;
     if (encoding == String::TWO_BYTE_ENCODING) {
-      offset = IntPtrAddFoldConstants(offset, offset);
+      offset = IntPtrAdd(offset, offset);
     }
-    offset = IntPtrAddFoldConstants(offset, IntPtrConstant(header));
+    offset = IntPtrAdd(offset, IntPtrConstant(header));
     return offset;
   }