[llvm] [RISCV] Handle fixed length vectors with exact VLEN in lowerINSERT_SUBVECTOR (PR #84107)

[Philip Reames via llvm-commits]

https://github.com/preames commented:

This looks reasonable likely to be correct.  I've added a couple small comments on style to make things easier to follow, but I don't see any major problems.

Note to future self - I've glanced at code only.  Need to go through the test diffs carefully to see if that raises any questions.

https://github.com/llvm/llvm-project/pull/84107