[gardening] Fix temp variable intptr_t type to match offset uint32_t.

Follow-up to 52dc797
Fixes failed build https://ci.chromium.org/ui/p/dart/builders/ci.sandbox/vm-aot-linux-debug-simarm_x64/4968/overview

TEST=ci

Change-Id: Ifea13d795638e2ba7b6a1e6d50f3e91f012945cf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/428841
Commit-Queue: Alexander Aprelev <[email protected]>
Reviewed-by: Siva Annamalai <[email protected]>

Author: aam

runtime/vm/object_graph.cc

@@ -134,7 +134,7 @@ class ObjectSlots {
       // the visitors will visit them but we won't emit the field description in
       // the heap snapshot).
       if (contains_only_tagged_words && (slots->length() > 0)) {
-        intptr_t expected_offset = (*slots)[0].offset;
+        auto expected_offset = (*slots)[0].offset;
         for (auto& slot : *slots) {
           ASSERT_EQUAL(slot.offset, expected_offset);
           expected_offset += kCompressedWordSize;