[turbofan] Be more consistent about Smi comparisons.

This changes SimplifiedLowering to be more consistent when chosing Smi
representation as input for Number comparisons. We already had some
isolated logic for doing (speculative) Number comparisons on Smis, but
only in the case where that decision was based on type feedback, not on
information already present in the graph.

Bug: v8:7703
Change-Id: I25370ade630917675a6ac79b5ae6a8afd253dfc7
Reviewed-on: https://chromium-review.googlesource.com/1196422Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55518}

src/compiler/representation-change.cc

@@ -913,12 +913,15 @@ const Operator* RepresentationChanger::Int32OverflowOperatorFor(
 const Operator* RepresentationChanger::TaggedSignedOperatorFor(
     IrOpcode::Value opcode) {
   switch (opcode) {
+    case IrOpcode::kNumberLessThan:
     case IrOpcode::kSpeculativeNumberLessThan:
       return machine()->Is32() ? machine()->Int32LessThan()
                                : machine()->Int64LessThan();
+    case IrOpcode::kNumberLessThanOrEqual:
     case IrOpcode::kSpeculativeNumberLessThanOrEqual:
       return machine()->Is32() ? machine()->Int32LessThanOrEqual()
                                : machine()->Int64LessThanOrEqual();
+    case IrOpcode::kNumberEqual:
     case IrOpcode::kSpeculativeNumberEqual:
       return machine()->Is32() ? machine()->Word32Equal()
                                : machine()->Word64Equal();