[llvm] [RISCV][MCA] Pick the correct VPseudo sched class for indexed memory operation (PR #128978)

[Pengcheng Wang via llvm-commits]

https://github.com/wangpc-pp approved this pull request.

LGTM and please rebase after @topperc's commits.
Also, I think we need to add some sections about the MC/MCA support in RVV's LLVM doc of implementation details, it is becoming more and more complicated. WDYT? cc @mshockwave @luke957 @michaelmaitland 

https://github.com/llvm/llvm-project/pull/128978