Go back to typing the "first field map" as a field

This is a temporary change to minimize diffs.
The issue is that we need to update both the
reading and writing code at once if we want
to change the types of the maps without some
regressions due to new artificial mismatches.

src/coreclr/jit/valuenum.cpp

@@ -7795,8 +7795,8 @@ void Compiler::fgValueNumberAssignment(GenTreeOp* tree)
 
                             // Construct the "field map" VN. It represents memory state of the first field
                             // of all objects on the heap. This is our primary map.
-                            ValueNum fldMapVN = vnStore->VNForMapSelect(VNK_Liberal, TYP_REF, fgCurMemoryVN[GcHeap],
-                                                                        firstFieldSelectorVN);
+                            ValueNum fldMapVN = vnStore->VNForMapSelect(VNK_Liberal, firstFieldType,
+                                                                        fgCurMemoryVN[GcHeap], firstFieldSelectorVN);
 
                             ValueNum firstFieldValueSelectorVN = ValueNumStore::NoVN;
                             if (obj != nullptr)