[RISC-V] vector segment load/store width as a riscv_tune_param

[Greg McGary]

I am revisiting an effort to make the number of lanes for vector segment
load/store a tunable parameter.

A year ago, Robin added minimal and not-yet-tunable
common_vector_cost::segment_permute_[2-8]

Some issues & questions:

* Since this pertains only to segment load/store, why is the word "permute"
  in the name?

* Nit: why are these defined as individual members rather than an array
  referenced as segment_permute[NF-2]?

* I implemented tuning as a simple threshold for max NF where segment
  load/store is profitable. Test cases for vector segment store pass, but
  tests for load fail. I found that common_cost_vector::segment_permute is
  properly honored in the store case, but not even inspected in the load
  case. I will need to spelunk the autovec cost model. Clues are welcome.

G