[turbofan][x64] Reduce reg-to-reg moving instruction for LoadRootsRegister IR

This results in a roughly 10KB reduction in snapshot_blob.bin on x64.

Change-Id: I72aab2db4e3b2a896f624c3c2afc1ac2e9610e23
Reviewed-on: https://chromium-review.googlesource.com/981911Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Kanghua Yu <kanghua.yu@intel.com>
Cr-Commit-Position: refs/heads/master@{#52264}

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

@@ -178,6 +178,17 @@ class X64OperandGenerator final : public OperandGenerator {
         }
       }
     }
+    if (operand->InputCount() == 2) {
+      Node* left = operand->InputAt(0);
+      Node* right = operand->InputAt(1);
+      if (left->opcode() == IrOpcode::kLoadRootsPointer &&
+          right->opcode() == IrOpcode::kInt64Constant) {
+        int64_t offset = OpParameter<int64_t>(right->op());
+        DCHECK(is_int32(offset));
+        inputs[(*input_count)++] = TempImmediate(static_cast<int32_t>(offset));
+        return kMode_Root;
+      }
+    }
     BaseWithIndexAndDisplacement64Matcher m(operand, AddressOption::kAllowAll);
     DCHECK(m.matches());
     if (m.displacement() == nullptr || CanBeImmediate(m.displacement())) {