RFR: 8263642: javac emits duplicate checkcast for first bound of intersection type in cast [v3]

[Guoxiong Li]

> Hi all,
> 
> Before the phase `desugar`, the demo `(C1 & I1) o` has **one** type-cast sub-tree, `JCTypeIntersection C1&I1`, at the syntax tree. After the phase `desugar`, the `(C1 & I1) o` becomes **three** type-cast sub-trees: `JCTypeIntersection C1&I1`, `I1`, `C1`. So at phase `Gen`, `javac` generates **three** `checkcast` according to the **three** type-cast sub-trees which causes this bug.
> 
> This patch doesn't generate `checkcast` when the type is `JCTypeIntersection` so that the problem can be solved. And a corresponding test case is added.
> 
> Another way to solve this issue is that the `TransTypes` of  the `desugar` should be modified and **two** type-cast sub-trees should be generated instead of **three**. But this way may change more code than my original patch and may cause other regression.
> 
> Thank you for taking the time to review.
> 
> Best Regards.
> --Guoxiong

Guoxiong Li has updated the pull request incrementally with one additional commit since the last revision:

  Encapsulate common code to a method

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3399/files
  - new: https://git.openjdk.java.net/jdk/pull/3399/files/0806d1bd..5f4f5bb7

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3399&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3399&range=01-02

  Stats: 19 lines in 1 file changed: 5 ins; 8 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3399.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3399/head:pull/3399

PR: https://git.openjdk.java.net/jdk/pull/3399