[turbofan][x64] Try harder to use memory addressing modes.

When we don't have a base, and the displacement returned by the
BaseWithIndexAndDisplacement64Matcher cannot be encoded as immediate,
we can still try to utilize the scale factor matching by just using
the displacement as base. This happens when we do indexed memory
accesses to known addresses.

R=jarin@chromium.org

Review-Url: https://codereview.chromium.org/2504123002
Cr-Commit-Position: refs/heads/master@{#41016}

src/compiler/x64/instruction-selector-x64.cc

@@ -154,10 +154,18 @@ class X64OperandGenerator final : public OperandGenerator {
     }
     BaseWithIndexAndDisplacement64Matcher m(operand, AddressOption::kAllowAll);
     DCHECK(m.matches());
-    if ((m.displacement() == nullptr || CanBeImmediate(m.displacement()))) {
+    if (m.displacement() == nullptr || CanBeImmediate(m.displacement())) {
       return GenerateMemoryOperandInputs(
           m.index(), m.scale(), m.base(), m.displacement(),
           m.displacement_mode(), inputs, input_count);
+    } else if (m.base() == nullptr &&
+               m.displacement_mode() == kPositiveDisplacement) {
+      // The displacement cannot be an immediate, but we can use the
+      // displacement as base instead and still benefit from addressing
+      // modes for the scale.
+      return GenerateMemoryOperandInputs(m.index(), m.scale(), m.displacement(),
+                                         nullptr, m.displacement_mode(), inputs,
+                                         input_count);
     } else {
       inputs[(*input_count)++] = UseRegister(operand->InputAt(0));
       inputs[(*input_count)++] = UseRegister(operand->InputAt(1));