[code-reflection] RFR: BytecodeGenerator cleanup and switch support [v7]

[Adam Sotona]

On Tue, 2 Dec 2025 10:25:34 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 two additional commits since the last revision:
> 
>  - removed obsolete block
>    
>  - LoweringTransform refactored out to jdk.incubator.code.bytecode.impl package

@mabbay nice job!

Synthetic type factory now looks like this:

  public static jdk.incubator.code.extern.ExternalizedTypeElement $exterType(int);
    descriptor: (I)Ljdk/incubator/code/extern/ExternalizedTypeElement;
    flags: (0x0009) ACC_PUBLIC, ACC_STATIC
    Code:
      stack=5, locals=9, args_size=1
         0: iload_0
         1: tableswitch   { // 0 to 146     
         ...

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

PR Review: https://git.openjdk.org/babylon/pull/706#pullrequestreview-3529999325