[vm/compiler] Avoid unnessary checks on 64-bit

Rationale:
Follow up from two previous CLs. Use of 64-bit macro
(requested by Martin) and avoid unnessary check on
64-bit platforms (requested by Alex)

https://github.com/dart-lang/sdk/issues/37868
https://github.com/dart-lang/sdk/issues/37895

Change-Id: I80e6825953090f5079377d0452b70e48c5be92a2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/114550
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Aart Bik <ajcbik@google.com>

runtime/vm/compiler/backend/type_propagator.cc

@@ -662,7 +662,7 @@ CompileType CompileType::Int() {
 }
 
 CompileType CompileType::Int32() {
-#if defined(TARGET_ARCH_X64) || defined(TARGET_ARCH_ARM64)
+#if defined(TARGET_ARCH_IS_64_BIT)
   return FromCid(kSmiCid);
 #else
   return Int();

runtime/vm/compiler/graph_intrinsifier.cc

@@ -280,11 +280,15 @@ static bool IntrinsifyArraySetIndexed(FlowGraph* flow_graph,
   switch (array_cid) {
     case kTypedDataUint8ClampedArrayCid:
     case kExternalTypedDataUint8ClampedArrayCid:
-      // Clamping operations need the exact value for proper operations.
-      // All others get away with truncating even non-smi values.
+#if defined(TARGET_ARCH_IS_32_BIT)
+      // On 32-bit architectures, clamping operations need the exact value
+      // for proper operations. On 64-bit architectures, kUnboxedIntPtr
+      // maps to kUnboxedInt64. All other situations get away with
+      // truncating even non-smi values.
       builder.AddInstruction(new CheckSmiInstr(new Value(value), DeoptId::kNone,
                                                builder.TokenPos()));
       FALL_THROUGH;
+#endif
     case kTypedDataInt8ArrayCid:
     case kTypedDataInt16ArrayCid:
     case kTypedDataUint8ArrayCid: