[llvm] [RISCV] Check isFixedLengthVector before calling getVectorNumElements in getSingleShuffleSrc. (PR #125455)

[Philip Reames via llvm-commits]

https://github.com/preames approved this pull request.

LGTM

Can you add a comment to the test file describing the case being exercised?  Your explanation from the review description would make a good starting point.

https://github.com/llvm/llvm-project/pull/125455