[csa] Optimize CodeStubAssembler::DecodeWord

Only use shifts in case masking has no effects.

Change-Id: I0b8b759ce9c9689917745e81345ceb3e16e994c3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2875085Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Patrick Thier <pthier@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74472}

src/codegen/code-stub-assembler.cc

@@ -7721,15 +7721,25 @@ TNode<Object> CodeStubAssembler::OrdinaryToPrimitive(
 TNode<Uint32T> CodeStubAssembler::DecodeWord32(TNode<Word32T> word32,
                                                uint32_t shift, uint32_t mask) {
   DCHECK_EQ((mask >> shift) << shift, mask);
-  return Unsigned(Word32And(Word32Shr(word32, static_cast<int>(shift)),
-                            Int32Constant(mask >> shift)));
+  if ((std::numeric_limits<uint32_t>::max() >> shift) ==
+      ((std::numeric_limits<uint32_t>::max() & mask) >> shift)) {
+    return Unsigned(Word32Shr(word32, static_cast<int>(shift)));
+  } else {
+    return Unsigned(Word32And(Word32Shr(word32, static_cast<int>(shift)),
+                              Int32Constant(mask >> shift)));
+  }
 }
 
 TNode<UintPtrT> CodeStubAssembler::DecodeWord(TNode<WordT> word, uint32_t shift,
                                               uintptr_t mask) {
   DCHECK_EQ((mask >> shift) << shift, mask);
-  return Unsigned(WordAnd(WordShr(word, static_cast<int>(shift)),
-                          IntPtrConstant(mask >> shift)));
+  if ((std::numeric_limits<uintptr_t>::max() >> shift) ==
+      ((std::numeric_limits<uintptr_t>::max() & mask) >> shift)) {
+    return Unsigned(WordShr(word, static_cast<int>(shift)));
+  } else {
+    return Unsigned(WordAnd(WordShr(word, static_cast<int>(shift)),
+                            IntPtrConstant(mask >> shift)));
+  }
 }
 
 TNode<Word32T> CodeStubAssembler::UpdateWord32(TNode<Word32T> word,