Revert "[turbofan] Be more consistent about Smi comparisons."

This reverts commit 0aac3884.

Reason for revert: Various performance regressions (Blink and JSTests).

Original change's description:
> [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/1196422
> Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
> Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#55518}

TBR=jarin@chromium.org,bmeurer@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: v8:7703, chromium:881758, chromium:881759, chromium:881760
Change-Id: I4af9c0b0ad37dea17ceabeb721c92a7cc978987a
Reviewed-on: https://chromium-review.googlesource.com/1213185Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55720}

src/compiler/representation-change.cc

@@ -913,15 +913,12 @@ 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();