VM: Fix incorrect canonicalization rule for `UnboxedIntConverterInstr`.

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
diff --git a/runtime/vm/intermediate_language.cc b/runtime/vm/intermediate_language.cc
index 4d781eb5b752ae4f7831761efd799a0690177aad..0b5f48ed37ddae3458a12351e93b897dbc115213 100644
--- a/runtime/vm/intermediate_language.cc
+++ b/runtime/vm/intermediate_language.cc
@@ -2310,6 +2310,11 @@ Definition* UnboxedIntConverterInstr::Canonicalize(FlowGraph* flow_graph) {
       value()->definition()->AsUnboxedIntConverter();
   if ((box_defn != NULL) && (box_defn->representation() == from())) {
     if (box_defn->from() == to()) {
+      // Do not erase truncating convertions from 64-bit value to 32-bit values
+      // because such convertions erase upper 32 bits.
+      if ((box_defn->from() == kUnboxedMint) && box_defn->is_truncating()) {
+        return this;
+      }
       return box_defn->value()->definition();
     }