[RISCV] Generaize reduction tree matching to all integer reductions (#68014) (reapply)

This was reverted in 824251c9 exposed by this change in a previous patch.  Fixed in 199cbec9.  Original commit message follows.

This builds on the transform introduced in
https://github.com/llvm/llvm-project/pull/67821, and generalizes it for
all integer reduction types.

A couple of notes:
* This will only form smax/smin/umax/umin reductions when zbb is
enabled. Otherwise, we lower the min/max expressions early. I don't care
about this case, and don't plan to address this further.
* This excludes floating point. Floating point introduces concerns about
associativity. I may or may not do a follow up patch for that case.
* The explodevector test change is mildly undesirable from a clarity
perspective. If anyone sees a good way to rewrite that to stablize the
test, please suggest.