[flow analysis] Simplify handling of post increment/decrement.

This change makes two changes to how flow analysis works for post
increment/decrement operations.

Firstly, in the anaylzer implementation, the check against
`inference_update_4` is removed. Previously, the analyzer called
`FlowAnalysis.postIncDec` when `inference_update_4` was enabled, and
`FlowAnalysis.write` when it was disabled. This was unnecessary, since
that language feature had no effect on the flow analysis of post
increment/decrement. Furthermore, it was a violation of separation of
concerns, because even if that language feature _had_ had an effect on
the flow analysis of post increment/decrement, it would have been the
job of flow analysis to implement that effect, not the
client. Fortunately, `FlowAnalysis.write` happens to have the same
behavior as `FlowAnalysis.postIncDec` when `inference_update_4` is
disabled, so there is no behavioral change; this is purely a clean-up.

Secondly, the return type of `FlowAnalysis.postIncDec` is changed from
`ExpressionInfo?` to `void`, and the calls to `storeExpressionInfo`
are removed from call sites. Previously, `FlowAnalysis.postIncDec`
always returned `null`, so again, there is no behavioral change.

Note that these changes do not affect the front_end in any way, since
it de-sugars `x++` and `x--` to equivalent "let" expressions before
invoking flow analysis. Those let expressions, fortunately, have the
same flow analysis effect as `FlowAnalysis.postIncDec`. Thanks to the
test added in https://dart-review.googlesource.com/c/sdk/+/482342, we
have enough language test coverage to confirm this.

Change-Id: I6a6a6964253394c71e6bc72d801d9044b1b1ae30
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/482541
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>