[code-reflection] RFR: BytecodeGenerator cleanup and switch support [v5]

[Adam Sotona]

On Mon, 1 Dec 2025 17:07:12 GMT, Adam Sotona <asotona at openjdk.org> wrote:

>> The current `BytecodeGenerator` implementation assumes that code models have already been lowered before generation. Both `SwitchExpressionOp` and `SwitchStatementOp` with all-constant labels are examples of operations that can be lowered an alternative way, allowing `BytecodeGenerator` to produce more efficient bytecode using `tableswitch` and `lookupswitch` instructions. 
>> 
>> The proposed change shifts the responsibility for lowering these operations into the `BytecodeGenerator` itself, enabling selective lowering during bytecode generation. Consequently, `OpBuilder` and `BytecodeGenerator` tests have been updated to avoid invoking the lower transformation step before the `BytecodeGenerator` is called. 
>> 
>> A new internal operation, `ConstantLabelSwitchOp`, has been introduced. This acts as an intermediate representation for lowered `SwitchExpressionOp` and `SwitchStatementOp` instances with constant labels. 
>> 
>> Lowering of `SwitchExpressionOp` and `SwitchStatementOp` into `ConstantLabelSwitchOp` is also a part of this PR.
>> 
>> This PR also makes minor refinements to the BytecodeGenerator post-processing transformations.
>
> Adam Sotona has updated the pull request incrementally with one additional commit since the last revision:
> 
>   test update

@mabbay  The custom lowering transformation is probably worth to take out of the `BytecodeGenerator` into a standalone class in the `jdk.incubator.code.bytecode.impl` package.

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PR Comment: https://git.openjdk.org/babylon/pull/706#issuecomment-3597825167