Fix typos in comments across Venom analysis and codegen

[sashaodessa]

Description:

• Summary: Correct minor spelling mistakes in comments only; no functional changes.
• Changes:
  • vyper/venom/analysis/available_expression.py — “necessery” → “necessary”
  • vyper/venom/passes/algebraic_optimization.py — grammar in iszero comment
  • vyper/venom/venom_to_assembly.py — “specialy” → “specially”

[sashaodessa]

@harkal