fix(bytecode): improve analyze_legacy padding for SWAPN/DUPN/EXCHANGE

[rakita]

Summary

• Track both prev_opcode and opcode in the analysis loop to handle edge cases
• Add padding when opcodes with immediates (SWAPN, DUPN, EXCHANGE) have their immediate byte appear as STOP
• Example: [SWAPN, STOP] is padded to [SWAPN, STOP, STOP] because the STOP is consumed as the immediate operand

Test plan

• Added test test_bytecode_ends_with_immediate_opcode_and_stop_requires_padding covering SWAPN, DUPN, and EXCHANGE
• All existing tests pass

[rakita]

Fix for analysis in #3223

[rakita]

Waiting fir CI, lgtm

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will improve performance by 3.12%

_{Comparing jumpa (15176b8) with main (b437e0f)¹}

Summary

⚡ 1 improved benchmark
✅ 172 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	transfer	14.6 µs	14.2 µs	+3.12%

Footnotes

1. No successful run was found on main (b3a6913) during the generation of this report, so b437e0f was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩