Fix T2C crash from unterminated LLVM basic blocks

[jserv]

When t2c_trace_ebb() compiles a block whose last instruction is a non-branching, non-terminal instruction (e.g., addi, lw, sw) with no branch_taken/branch_untaken pointers, no LLVM terminator was emitted. This produced malformed IR that crashed LLVMRunPasses during the LowerExpectIntrinsicPass.

This occurs when a block ends at a page boundary or instruction limit before block chaining is established, or when a linked block has been evicted from the cache.

This adds a safety check at the end of t2c_trace_ebb() using LLVMGetBasicBlockTerminator() to detect unterminated blocks. When found, store block->pc_end to rv->PC, flush the cycle counter, and emit ret void to return control to the interpreter.

Close #718

---

Summary by cubic

Fixes a crash during LLVMRunPasses (LowerExpectIntrinsicPass) by ensuring every compiled basic block is properly terminated. If a block ends without a branch or a resolvable target, we now return to the interpreter safely.

• Bug Fixes
  • Detect unterminated LLVM basic blocks with LLVMGetBasicBlockTerminator.
  • When missing: write block->pc_end to rv->PC, flush the cycle counter, and emit ret void.
  • Prevents malformed IR when a block ends at a page boundary, hits the instruction limit, or its linked block was evicted.

^{Written for commit 9b56311. Summary will update on new commits.}