[compiler] Add/fix some comments.

Also put an UNREACHABLE into an impossible case in
NumberOpFromSpeculativeNumberOp.

R=jarin@chromium.org

Bug: 
Change-Id: I681b7bc58de5038497667cb48fdcd79a73abe1c2
Reviewed-on: https://chromium-review.googlesource.com/819415Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49998}

src/compiler/js-typed-lowering.cc

@@ -308,7 +308,8 @@ class JSBinopReduction final {
       case IrOpcode::kSpeculativeNumberLessThanOrEqual:
         return simplified()->NumberLessThanOrEqual();
       case IrOpcode::kSpeculativeNumberAdd:
-        return simplified()->NumberAdd();
+        // Handled by ReduceSpeculativeNumberAdd.
+        UNREACHABLE();
       case IrOpcode::kSpeculativeNumberSubtract:
         return simplified()->NumberSubtract();
       case IrOpcode::kSpeculativeNumberMultiply:
@@ -594,6 +595,9 @@ Reduction JSTypedLowering::ReduceSpeculativeNumberBinop(Node* node) {
   if ((hint == NumberOperationHint::kNumber ||
        hint == NumberOperationHint::kNumberOrOddball) &&
       r.BothInputsAre(Type::NumberOrUndefinedOrNullOrBoolean())) {
+    // We intentionally do this only in the Number and NumberOrOddball hint case
+    // because simplified lowering of these speculative ops may do some clever
+    // reductions in the other cases.
     r.ConvertInputsToNumber();
     return r.ChangeToPureOperator(r.NumberOpFromSpeculativeNumberOp(),
                                   Type::Number());
@@ -999,6 +1003,7 @@ Reduction JSTypedLowering::ReduceJSToNumberOrNumeric(Node* node) {
     NodeProperties::ChangeOp(node, simplified()->PlainPrimitiveToNumber());
     return Changed(node);
   }
+  // TODO(neis): Reduce ToNumeric to ToNumber if input can't be BigInt?
   return NoChange();
 }
 

src/compiler/simplified-lowering.cc

@@ -1753,7 +1753,7 @@ class RepresentationSelector {
         // Handle the case when no int32 checks on inputs are necessary
         // (but an overflow check is needed on the output).
         if (BothInputsAre(node, Type::Signed32())) {
-          // If both the inputs the feedback are int32, use the overflow op.
+          // If both inputs and feedback are int32, use the overflow op.
           if (hint == NumberOperationHint::kSignedSmall ||
               hint == NumberOperationHint::kSigned32) {
             VisitBinop(node, UseInfo::TruncatingWord32(),