[RISCV] Generaize reduction tree matching to fp sum reductions (#68599)

This builds on the transform introduced in f0505c3d, and generalizes to
all integer operations in 45a334d3. This change adds support for floating
point sumation.

A couple of notes:
* I chose to leave fmaxnum and fminnum unhandled for the moment. They
have a slightly different set of legality rules.
* We could form strictly sequenced FADD reductions for FADDs without
fast math flags. As the ordered reductions are more expensive, I left
thinking about this as a future exercise.
* This can't yet match the full vector reduce + start value idiom. That
will be an upcoming set of changes.