[turbofan] fold repeated essentially-pure operations

This improves a pattern that appears in the SunSpider
base64 benchmark.

Change-Id: I1b5f4535d24d8a39cc480508d1e11c8e92e0135c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3654098Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Auto-Submit: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80629}

src/compiler/redundancy-elimination.cc

@@ -35,6 +35,14 @@ Reduction RedundancyElimination::Reduce(Node* node) {
     case IrOpcode::kCheckSmi:
     case IrOpcode::kCheckString:
     case IrOpcode::kCheckSymbol:
+    // These are not really check nodes, but behave the same in that they can be
+    // folded together if repeated with identical inputs.
+    case IrOpcode::kBigIntAdd:
+    case IrOpcode::kBigIntSubtract:
+    case IrOpcode::kStringCharCodeAt:
+    case IrOpcode::kStringCodePointAt:
+    case IrOpcode::kStringFromCodePointAt:
+    case IrOpcode::kStringSubstring:
 #define SIMPLIFIED_CHECKED_OP(Opcode) case IrOpcode::k##Opcode:
       SIMPLIFIED_CHECKED_OP_LIST(SIMPLIFIED_CHECKED_OP)
 #undef SIMPLIFIED_CHECKED_OP