[turbofan] Introduce an ExternalPointer type.

src/compiler/common-node-cache.h

Index: src/compiler/common-node-cache.h
diff --git a/src/compiler/common-node-cache.h b/src/compiler/common-node-cache.h
index 1f07703e72395192a0e911f4e3e75046f3a3754b..bce8d0f62ed814149017c73b1f23ef8c16e3f99f 100644
--- a/src/compiler/common-node-cache.h
+++ b/src/compiler/common-node-cache.h
@@ -45,6 +45,10 @@ class CommonNodeCache final {
 
   Node** FindExternalConstant(ExternalReference value);
 
+  Node** FindPointerConstant(intptr_t value) {
+    return pointer_constants_.Find(zone(), value);
+  }
+
   Node** FindNumberConstant(double value) {
     // We canonicalize double constants at the bit representation level.
     return number_constants_.Find(zone(), bit_cast<int64_t>(value));
@@ -73,6 +77,7 @@ class CommonNodeCache final {
   Int32NodeCache float32_constants_;
   Int64NodeCache float64_constants_;
   IntPtrNodeCache external_constants_;
+  IntPtrNodeCache pointer_constants_;
   Int64NodeCache number_constants_;
   IntPtrNodeCache heap_constants_;
   RelocInt32NodeCache relocatable_int32_constants_;