Replace manual untagging of smi with proper Smi methods

runtime/vm/intermediate_language_x64.cc

Index: runtime/vm/intermediate_language_x64.cc
diff --git a/runtime/vm/intermediate_language_x64.cc b/runtime/vm/intermediate_language_x64.cc
index e9db8d86521273ddf1c1dff6b307f853d81a45a8..24ba11602f38a04c5fae6e3e84830873c95b98c3 100644
--- a/runtime/vm/intermediate_language_x64.cc
+++ b/runtime/vm/intermediate_language_x64.cc
@@ -6070,8 +6070,7 @@ void ShiftMintOpInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
   if (locs()->in(1).IsConstant()) {
     // Code for a constant shift amount.
     ASSERT(locs()->in(1).constant().IsSmi());
-    const int64_t shift =
-        reinterpret_cast<int64_t>(locs()->in(1).constant().raw()) >> 1;
+    const int64_t shift = Smi::Cast(locs()->in(1).constant()).Value();
     ASSERT(shift >= 0);
     switch (op_kind()) {
       case Token::kSHR: