Fix LLVM builder leaks in T2C's cbeqz/cbnez/fuse12

[jserv]

The else branches in cbeqz, cbnez, and fuse12 handlers create LLVMBuilderRef objects that are used to emit RetVoid but never disposed, leaking heap-allocated LLVM objects per T2C compilation.

This adds LLVMDisposeBuilder() in each else branch, matching the pattern already used by the BRANCH_FUNC macro. Also add t2c_check_valid_blk() guard to cbeqz/cbnez branch conditions to prevent chaining to evicted or invalid blocks, aligning with BRANCH_FUNC and fuse12.

Close #722

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Summary by cubic

Fixes LLVMBuilderRef leaks in T2C’s cbeqz/cbnez/fuse12 and adds a guard to avoid chaining to invalid blocks. This reduces per-compile heap growth and aligns behavior with BRANCH_FUNC.

• Bug Fixes
  • Dispose LLVM builders after emitting RetVoid in else paths for cbeqz, cbnez, and fuse12 (LLVMDisposeBuilder).
  • Gate branch_taken/untaken in cbeqz/cbnez with t2c_check_valid_blk to prevent chaining to evicted/invalid blocks, matching BRANCH_FUNC.

^{Written for commit 7cd4602. Summary will update on new commits.}