[compiler] Simplify node matching in loop variable analyzer.

This removes a condition that seems to have become always true in the
cases we care about.

R=jarin@chromium.org

Bug: 
Change-Id: I3684db93ab7e498b7b7ac0b42945625c30fd1afa
Reviewed-on: https://chromium-review.googlesource.com/806225Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49883}

src/compiler/loop-variable-optimizer.cc

@@ -318,14 +318,8 @@ InductionVariable* LoopVariableOptimizer::TryGetInductionVariable(Node* phi) {
   }
 
   // TODO(jarin) Support both sides.
-  // XXX
-  if (arith->InputAt(0) != phi) {
-    if ((arith->InputAt(0)->opcode() != IrOpcode::kJSToNumber &&
-         arith->InputAt(0)->opcode() != IrOpcode::kSpeculativeToNumber) ||
-        arith->InputAt(0)->InputAt(0) != phi) {
-      return nullptr;
-    }
-  }
+  if (arith->InputAt(0) != phi) return nullptr;
+
   Node* incr = arith->InputAt(1);
   return new (zone())
       InductionVariable(phi, arith, incr, initial, zone(), arithmeticType);