[code-reflection] RFR: Fix the bug of pattern variable initialized with conversion

[Mourad Abbay]

Generating code model for code such as:
 ```@Reflect
    void m(int i) {
        if (i instanceof byte b) {

        }
    }
   ```
was causing the below error in the compiler:

java.lang.IllegalStateException: Operand of operation jdk.incubator.code.dialect.core.CoreOp$VarOp at 503d687a is not defined in tree: %result at 482bce4f
        at jdk.incubator.code/jdk.incubator.code.Block.bindOp(Block.java:764)
        at jdk.incubator.code/jdk.incubator.code.Block.insertOp(Block.java:743)
        at jdk.incubator.code/jdk.incubator.code.Block$Builder.op(Block.java:697)
        at jdk.incubator.code/jdk.incubator.code.internal.ReflectMethods$BodyScanner.scanPattern(ReflectMethods.java:1342)
...

The cause is that we were adding the op that perform the conversion to the block of the IfOp first body, making the value unreachable to the VarOp of the pattern variable.
The fix is from @PaulSandoz.

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

Commit messages:
 - Apply the fix

Changes: https://git.openjdk.org/babylon/pull/771/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=771&range=00
  Stats: 13 lines in 1 file changed: 7 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/babylon/pull/771.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/771/head:pull/771

PR: https://git.openjdk.org/babylon/pull/771