[RISCV] Make use of SHXADD instructions in RVV spill/reload code.

We can use SH1ADD, SH2ADD, SH3ADD to multipy by 3, 5, and 9 respectively.

We could extend this to 3, 5, or 9 multiplied by a power 2 by also
emitting a SLLI.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D124824