[code-reflection] RFR: Make sure method parameters keep their slots when lowering

[Hannes Greule]

Previously, unused method parameters didn't get a slot, resulting in either wrong results or bad classfiles.

I propose a simple fix: Make sure that all parameters of the entry block get a slot, in the order they are defined.
I didn't want to change too much code here, so I decided to add a boolean flag to indicate that even values without uses get a slot.
If you have a better idea or more plans on this, please let me know.

I also added multiple test cases showcasing the issue: One that results in the wrong value returned, one with a type mismatch resulting in a VerifyError, and one with a type mismatch due to being non-static, also causing a VerifyError (this is the case that made me discover this issue).

I ran the tests in `langtools/tools/javac/reflect/` and `java/lang/reflect/code/` to make sure they all (still) pass.

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Commit messages:
 - Make sure method parameters keep their slots when lowering

Changes: https://git.openjdk.org/babylon/pull/8/files
 Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=8&range=00
  Stats: 68 lines in 2 files changed: 67 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/babylon/pull/8.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/8/head:pull/8

PR: https://git.openjdk.org/babylon/pull/8