[code-reflection] RFR: Switch lowering fixes

[Adam Sotona]

This patch fixes `bytecode/impl/LoweringTransform` with unboxing selector, implicit default target leading to exit block and conversion of constant labels.
Unreflect runs of `TestSwitchExpressionOp` and `TestSwitchStatementOp` are now enabled and pass.

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Commit messages:
 - fixed converted constant labels
 - unboxing selector

Changes: https://git.openjdk.org/babylon/pull/804/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=804&range=00
  Stats: 28 lines in 3 files changed: 22 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/babylon/pull/804.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/804/head:pull/804

PR: https://git.openjdk.org/babylon/pull/804