[csa] Transform multiplications into left shifts

Turn cases where we are multiplying with a power of two into a left shift. We
hit this optimisation roughly 500 times in the snapshot.

Bug: 
Change-Id: Ibd3104a3dbe49f247a2d84db94891f6e3a897026
Reviewed-on: https://chromium-review.googlesource.com/763229Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Cr-Commit-Position: refs/heads/master@{#49306}

src/compiler/code-assembler.cc

@@ -465,12 +465,12 @@ TNode<WordT> CodeAssembler::IntPtrMul(SloppyTNode<WordT> left,
     if (is_right_constant) {
       return IntPtrConstant(left_constant * right_constant);
     }
-    if (left_constant == 1) {
-      return right;
+    if (base::bits::IsPowerOfTwo(left_constant)) {
+      return WordShl(right, WhichPowerOf2(left_constant));
     }
   } else if (is_right_constant) {
-    if (right_constant == 1) {
-      return left;
+    if (base::bits::IsPowerOfTwo(right_constant)) {
+      return WordShl(left, WhichPowerOf2(right_constant));
     }
   }
   return UncheckedCast<IntPtrT>(raw_assembler()->IntPtrMul(left, right));

test/unittests/compiler/code-assembler-unittest.cc

@@ -109,6 +109,20 @@ TARGET_TEST_F(CodeAssemblerTest, IntPtrMul) {
     Node* c = m.IntPtrMul(a, b);
     EXPECT_THAT(c, IsIntPtrConstant(500));
   }
+  // x * 2^CONST  => x << CONST
+  {
+    Node* a = m.Parameter(0);
+    Node* b = m.IntPtrConstant(1 << 3);
+    Node* c = m.IntPtrMul(a, b);
+    EXPECT_THAT(c, IsWordShl(a, IsIntPtrConstant(3)));
+  }
+  // 2^CONST * x  => x << CONST
+  {
+    Node* a = m.IntPtrConstant(1 << 3);
+    Node* b = m.Parameter(0);
+    Node* c = m.IntPtrMul(a, b);
+    EXPECT_THAT(c, IsWordShl(b, IsIntPtrConstant(3)));
+  }
 }
 
 TARGET_TEST_F(CodeAssemblerTest, WordShl) {