BytecodeGenerator cleanup and switch support

[asotona]

• BytecodeGenerator performs proprietary lowering transformation
• OpBuilder and tests adjusted to do not lower before calling BytecodeGenerator
• BytecodeGenerator.ConstantLabelSwitchOp is an internal lowered representation of JavaOp.JavaSwitchOp with constant labels, and it generates tableswitch or lookupswitch instruction
• post-processing transformations are slightly refactored

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Progress

• Change must not contain extraneous whitespace

Reviewing

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[openjdk]

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