[RISCV] Prune unneeded indexed load/store pseudo instructions.

We were creating more combinations of value and index lmul than
we needed.

I've copied the loop structure used here from VPseudoAMOEI with
all data sew values instead of just 32/64.

Similar can be done for segment loads/store.

Reviewed By: khchen

Differential Revision: https://reviews.llvm.org/D97008