[RISCV] Add caching to the gather/scatter to strided load/store conversion.

If we have multiple gather/scatter instructions using the same the
same strided address we would scalarize it multiple times. I guess
a later pass cleans this up, but I don't know if that's guaranteed.

This patch adds a cache to remember the scalarization we already
created for a previous gather/scatter.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D125326