[VPlan] Support symbolic stride replacement in convertToStridedAccesses

`@double_stride_ptr_iv` in `llvm/test/Transforms/LoopVectorize/RISCV/strided-accesses.ll` is a test case where a gather cannot be converted into a strided load.

The root cause seems to be that `replaceSymbolicStrideSCEV` in `LoopAccessInfo` is able to leverage loop versioning assumptions, which allows certain non-constant strides to be treated as constant-stride accesses. However, because `VPlanTransforms::convertToStridedAccesses` does not currently apply `replaceSymbolicStrideSCEV`, the stride is evaluated as non-constant, preventing the transformation into a strided load.

Start with llvm/test/Transforms/LoopVectorize/RISCV/strided-accesses.ll, especially @double_stride_ptr_iv, and trace VPlanTransforms::convertToStridedAccesses alongside LoopAccessInfo::replaceSymbolicStrideSCEV. Check how loop versioning assumptions affect the stride evaluation. Done means the symbolic stride is recognized so the gather can be converted into a strided load, with the relevant test passing.

Written by the indexing model from the issue text.