[RISCV] Improve SiFive7 for loads and stores

* Unit-stride loads and stores can operate at the full bandwidth of the
memory pipe. The memory pipe is DLEN bits wide.

* Strided loads and stores operate at one element per cycle and should
be scheduled accordingly.

* Indexed loads and stores operate at one element per cycle, and they
stall the machine until all addresses have been generated, so they
cannot be scheduled.

* Unit stride seg2 load is number of DLEN parts

* seg3-8 are one segment per cycle, unless the segment is larger
than DLEN in which each segment takes multiple cycles.

Differential Revision: https://reviews.llvm.org/D153475