[llvm] [RISCV] Add codegen support for ri.vinsert.v.x and ri.vextract.x.v (PR #136708)

[Philip Reames via llvm-commits]

https://github.com/preames commented:

A bit of context for reviewers on the upcoming patches in this area.  This is the first in a small series, but at the moment, it's a quite small series.  I've got a buildvector lowering change (very small), want to rework the extract path to use vmv.s.x for index zero, and one possible bugfix I have to either extract a test for, or convince myself it's no longer relevant.  

I might later explore the fold memory operand and rematerialization hooks, but that'll be a bit down the line.  

https://github.com/llvm/llvm-project/pull/136708