[Hexagon] Track type locally in HexagonVectorCombine (#179066)
Replace getAllocatedType calls with tracked types from alloca creation.
The types are known at the CreateAlloca call sites, so we track them
locally instead of re-querying through getAllocatedType, to facilitate
someday possibly removing getAllocatedType from the API of AllocaInst.
Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>